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  1. #1
    Left The Driveway
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    Replacement King Mattress

    First Post, so be gentle.

    We picked up our 2019 399TH at the end of January and started living in it full time on the 12th of Feb. Not even a month in and the King Mattress already has indentations from the two of us. I would estimate of the 5.5" the mattress is supposed to be, where we sleep is around 3" now.

    My problem is the size of the mattress. The tag says it is a 72" x 80". However, there seems to be a "cut" on the right hand, headboard side. Also, the center of the mattress against the headboard has about a 2" gap from the stock mattress. I'm assuming this shape is common for all Momentums except the 376 and 397 which appear to have flat walls at the headboard. The rest are all off-center in the front cap.

    Has anyone replaced the stock mattress in their 399TH and if so, with what?

    We were looking at the sleep number r5 and possibly getting the King Radius, but even the sleep number store doesn't know the dimensions.

    If you have had success with either sleep number option (RV King or RV Radius King) or any mattress for that matter, please let us know. I'm about to start sleeping in the garage.

    Any assistance would be greatly appreciated.

    Marty

  2. #2
    Long Hauler bertschb's Avatar
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    Welcome to the forum Marty!

    I've never owned a Momentum so I apologize for not answering your question as a Momentum owner. But, we did replace the mattress in our Reflection after using it for a couple of months. Like your factory mattress, ours quickly formed indentations. After our experience with the mattress in our Reflection, we replaced the mattress in our Solitude before we even used it. In both cases, we bought Sleep Number mattresses and love them. The next time we replace a mattress at home, it will be with a Sleep Number. The ability to dial in your preferred firmness really does work and they are built better than I expected for a glorified air mattress. Pretty cool design really.

    BTW, we didn't get the "RV" sized mattresses. Our mattresses fit fine on the platform. If the dimensions would have been way off, I would have just replaced the platform. It's just plywood and would be super easy to make it any size you want. Plus it would be cheaper to do that than get the RV specific mattress sizes which I believe are quite a bit more expensive than standard sizes.

    Good luck and I hope somebody with a 399TH chimes in...
    Brian & Kellie
    2020 Solitude 310GK-R, FBP, 1,460w solar, 540ah BBGC3, MORryde IS w/disc brakes
    2020 F-350 Platinum SRW Powerstroke Tremor, 60g TF fuel tank, Hensley BD3-F air bag hitch

    Previous setups:
    2019 Solitude 373FB-R, 2019 F-350 Platinum DRW Powerstroke, Hensley BD5 air bag hitch
    2016 Reflection 318RST, 2016 GMC 3500 Denali SRW Duramax, Hensley BD3 air bag hitch

  3. #3
    Seasoned Camper joesmainship's Avatar
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    I can't address the radius issue, but we replaced our king mattress with a Sleep Number R5 and we loved so much we bought a sleep number for the house. 72 X 80 is the correct dimension for the RV King size. Camping World puts them on sale a couple of times a year as does the SN store. You'll pay a little more at the SN store, but you'll have better warranty coverage.
    The Capt and the Admiral
    Joe & Marilyn
    2017 Solitude 384 GK-R
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  4. #4
    Long Hauler howson's Avatar
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    Quote Originally Posted by N&M_Weaver View Post
    We picked up our 2019 399TH...Any assistance would be greatly appreciated.
    Marty,
    I don't have a Momentum but installed a normal Queen in my 315RLTS. Probably not very helpful info...sorry. Hopefully someone with your rig will be along shortly with advice.

    Welcome to the forum!

    -Howard
    2017 Ford F-350 DRW 6.7L Platinum
    2019 315RLTS (purchased 16 Jul 18 from Campers Inn RV in Byron, GA)

  5. #5
    Paid my dues 😁 FT4NOW's Avatar
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    Welcome to the forum. I have a 399TH and replaced the mattress. We have the king option and purchased a 9" memory foam mattress from Amazon (I dont remember the brand). The mattress came with a zippered cover. We removed the cover and cut it to fit with a large bread knife using a sawing action. To get the template correct, I took a 12" ruler and taped a marker perpendicular to it. I moved the mattress centrepoint about 12" from the headboard and then ran the ruler along the wall. As the ruler runs along the wall it draws a line on the mattress, we then cut the corners along the line. We rotated the mattress 180 and did the same thing to allow for mattress rotation. When it was all done we put the zippered cover back on and it works great.

    Sent from my SM-G950U using Tapatalk
    2023 Momentum 398M-R
    2023 Ford F-450

    SOLD - 2021 Reflection 311BHS
    SOLD - 2017 Momentum 399TH

  6. #6
    Big Traveler arhayes's Avatar
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    We have a queen but were able to replace it with a Sleep Number standard queen. You pay a premium for the “RV size” unit. If measure the space available then go from there.

  7. #7
    Left The Driveway
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    Thanks All,

    Looks like I will have to get something custom made. I can't use a standard King because the width is only 72", if I go with the standard California King, it adds 4" to the length. The shortest side is 71" long on the platform so the bed would stick out 13" (eliminating the access to the bathroom) and a straight headboard cut leaves about 9" of a gap at the headboard. Not ideal.

    Thanks again for the help though.

    Marty

  8. #8
    Setting Up Camp gobama84's Avatar
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    Quote Originally Posted by N&M_Weaver View Post
    Thanks All,

    Looks like I will have to get something custom made. I can't use a standard King because the width is only 72", if I go with the standard California King, it adds 4" to the length. The shortest side is 71" long on the platform so the bed would stick out 13" (eliminating the access to the bathroom) and a straight headboard cut leaves about 9" of a gap at the headboard. Not ideal.

    Thanks again for the help though.

    Marty
    Check out Mattress Insider. Had one in a motorhome years ago. They also make custom mattresses. https://www.mattressinsider.com/rv-mattress.html
    Loyd - USMC Retired & Dean - SSA Retired
    Looking to Full Time as soon as we buy a 397TH
    Currently working as a Government Contractor

  9. #9
    Setting Up Camp SoCalMoto's Avatar
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    Quote Originally Posted by gobama84 View Post
    Check out Mattress Insider. Had one in a motorhome years ago. They also make custom mattresses. https://www.mattressinsider.com/rv-mattress.html

    FWIW - I am taking delivery of a 2019 399TH next week and have the King bed option. I called Grand Design and they advised the stock King bed is 70 x 80.
    Unfortunately, Mattress Insider doesn't offer a mattress in that size, at least I didn't see one listed on their site. It appears this site http://www.comfortcustombedding.com offers custom 70 x 80 mattresses and might be worth considering.

    I will post details of my mattress upgrade once I get my hands on the rig!

    -Garo
    - - - - -
    2019 Momentum 399TH Blue/Silver
    2018 RAM 3500 DRW Laramie

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  10. #10
    Setting Up Camp SoCalMoto's Avatar
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    Update: I called Mattress Insider and they will happily make any custom size mattress you want. If the size you want is not listed on their site, just call them at 888-488-1468.

    -Garo
    - - - - -
    2019 Momentum 399TH Blue/Silver
    2018 RAM 3500 DRW Laramie

    Toys
    2018 Polaris RZR Dynamix Turbo
    2018 CRF450RX
    2018 YZ450FX
    2016 YZ250

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