31MB Actual tank sizes

[Appfan]

Hey folks,
New owner here and I'm curious if anyone knows the actual size of the individual tanks in the 31MB. They come equipped with (2) gray tanks, 1 black and 1 white.

The specs say the following:

• 60 gal white
• 75 gal gray
• 43 gal black

Where I'm going with this is I know that some manufacturers include the hot water heater in the white capacity calculations. Also, I'm curious if they gray tanks are evenly split or if one is larger than the other.

Cheers!

[jh.xsnrg]

On our 2019 Imagine, the hot water heater capacity is included in the fresh water tank size. Not sure about weight.

[timshoemaker]

Hey folks,
New owner here and I'm curious if anyone knows the actual size of the individual tanks in the 31MB. They come equipped with (2) gray tanks, 1 black and 1 white.

The specs say the following:

• 60 gal white
• 75 gal gray
• 43 gal black

Where I'm going with this is I know that some manufacturers include the hot water heater in the white capacity calculations. Also, I'm curious if they gray tanks are evenly split or if one is larger than the other.

Cheers!

I'm thinking of purchasing a 31MB as well and am trying to figure out this same question. Did you ever figure it out? It looks like there is only one sewer outlet, but there is one gray handle in the compartment next to the black handle and another gray handle under the camper. I'm curious how you dump the tanks on this unit too.

[timshoemaker]

This is what I just got back from my dealer, who said they got this info from their Grand Design guy:

75 gallon total capacity = 43 bathroom + 32 kitchen and one sewer termination


Now my question is, how do I combine the capacity. Just put a gate value at the sewer termination and open both gray valves? Is it that simple?

[Canyonlight]

This is what I just got back from my dealer, who said they got this info from their Grand Design guy:

75 gallon total capacity = 43 bathroom + 32 kitchen and one sewer termination


Now my question is, how do I combine the capacity. Just put a gate value at the sewer termination and open both gray valves? Is it that simple?

That is what most folks do and have done for years. The only potential consideration is that if your black tank valve leaks, that black water could be or will be mixing with your grey water/tanks/etc. Everyone has their propensity for risk and determination of how big of a concern this is to you in your situation but just something to be aware of in case you may not have considered.

Dan

[Appfan]

This is what I just got back from my dealer, who said they got this info from their Grand Design guy:

75 gallon total capacity = 43 bathroom + 32 kitchen and one sewer termination


Now my question is, how do I combine the capacity. Just put a gate value at the sewer termination and open both gray valves? Is it that simple?

Hey Tim,
Regardless of whether you "combine" they gray tanks or not, you'll definitely want a gate valve at the sewer termination. No need to "surprise yourself" the next time you go to connect your sewer hose. I've seen too many posts lately, here and elsewhere, of folks getting a nasty surprise when they take the cap off.

[timshoemaker]

Makes sense. Is this the best one:

https://www.amazon.com/Valterra-T58-...gateway&sr=8-3

[Appfan]

I'm, by no means, a long-time RVer but that's the one I've got. I also put a pipe clamp around it to make sure it doesn't disappear while on the road. Not tight enough to crack anything, just super snug.

[jdzielak]

I'm not sure about the exact tank sizes on my 31MB or if the WH is included in the calculation. Someone filling their fresh tank with a water meter should be able to confirm this by how much water goes in it before it overflows. I'd also be interested to know this. For dry camping I'm planning on expanding my fresh capacity by putting a 50G water tank in the back of my truck and using the sanitize function in the nautilus to transfer from the auxiliary tank to the fresh tank in the camper.

I can confirm that there is a separate valve for the gray tank serving the bathroom and the separate gray tank serving the kitchen. There is only one drain. The black and both gray tanks drain into this common sewer line for a single point of connection to your sewer hookup or dump station (which is convenient). The valves appear to be at the tank, which is heated, for freeze protection and have remote releases via cable. By combining the gray tanks with the method discussed above you would be subject to this line freezing in the winter unless you heat traced and insulated it.

[Canyonlight]

I'm not sure about the exact tank sizes on my 31MB or if the WH is included in the calculation.

Attached GDRV web page for the 31MB specs. section where tank sizes are stated as noted in post #4 . Believe this is in your owners manual/owners packet of information as well:

https://www.granddesignrv.com/showro...loorplans/31mb

The water heater capacity is included as part of the fresh water tank size. Believe this is also covered in the owners manual/information.

Dan