Was out this past weekend in our 21 G-Series, and our sink backed up on the second day, so I took the plumbing apart under the sink and didn't find any sort of clog. Being that we were in the desert and hadn't been using the sink for more than a day I figured it was safe to crack open the grey tank to see if for some reason maybe the tank was still partially full from our previous trip or something? I cracked the grey and only about 2-3 gallons came out. We tried using a small Milwaukee leaf blower to blow air in the plumbing from the sink, tried a small piece of wire to try and see if we could find a clog at the sink end and the drain side, but we were unable to sort anything out and at this point the sink is still backing up! Really lost with this and any help is much appreciated!
21G with backed up sink and open grey tank valve?!!
I would get a longer snake
If it isn’t draining it is clogged with something
Hard to imagine with what but only you would know that
My mother when she browns hamburger dumps the liquid down her drain. I only realized this when she said her sink was plugged
She doesn’t do it anymore
Endoscopes are dirt cheap on Amazon as well
They are always a good thing to have in the toolbox
2021 Imagine 2400 BH
2018 GMC Sierra 4x4 crew
If it isn’t draining it is clogged with something
Hard to imagine with what but only you would know that
My mother when she browns hamburger dumps the liquid down her drain. I only realized this when she said her sink was plugged
She doesn’t do it anymore
Endoscopes are dirt cheap on Amazon as well
They are always a good thing to have in the toolbox
2021 Imagine 2400 BH
2018 GMC Sierra 4x4 crew
Try removing the air admittance valve in the cabinet under the sink. If this is stuck it can restrict the drain flow.
Calbar
Senior Member
It could also be a bad air admittance valve that is causing a vacuum in the plumbing and not allowing the water to drain properly. In order for plumbing to drain you need air behind it which is one reason for plumbing vents and air admittance valve. There will be one under the counter likely behind the sink plumbing. It will look something like the one in the attached link.
https://www.dkhardware.com/39012-1-1-2-in-abs-ptc-in-line-round-trap-bend-cheater-vent-product-3359927.html?gclid=EAIaIQobChMIt_-0-sqy7wIVko7ICh0GEwUiEAQYByABEgLNffD_BwE
Rob
Can't believe I missed this. The way the trailer dump valves were explained I had no idea there were not only two grey tanks, but also two grey tank valves! I swore it was a shared valve! Oh man, I feel like a complete newbie, but it was good to get home from work and find that there was indeed a second grey tank valve, and it was completely frozen! Went to work with some PB Blaster and we're back in business. Thanks for pointing out the not so obvious to me!
Good to hear. I would have mentioned this as well but noticed the trailer was a 2019 and figured you would have used that valve 100 times by now
Didn’t know
2021 Imagine 2400 BH
2018 GMC Sierra 4x4 crew
Don't blame ya!! We bought it used just before the New Year and had only used it once for 2 nights before our recent trip last weekend and the previous owner only used it once!
Tommyp228
Advanced Member
Don’t fell bad, just got back from our first outing in our 21G. Only drained on gray tank. Didn’t realize they had 2. Is the rear drain just the shower and bathroom sink? Going to have to get a bucket and drain a little at a time.
Gotta admit, that does make me feel a little better!
Now that I'm an expert, I can tell you that the grey tank handle that is next to the black tank handle is for the shower and bathroom sink grey tank. The rear grey tank handle that is near the onboard fuel tank, is to drain the kitchen sink grey tank.
Grunt Doc
New Member
Just had the same issue with my new 21G. Was getting just as frustrated as Zeke was. After reading the posts on this thread, I found a second Grey tank drain valve. Problem solved! Why do I Not remember any mention in the literature about this? And the salesman certainly never showed or mentioned this. Either way, I found the answer to my problem here, by folks who clearly know what their doing. Thanks everyone for the info!
Almost Retired
New Member
- Joined
- Jul 15, 2021
- Messages
- 1
Just picked up a '19 31MB. Does this have a 2nd valve also? Took out the P trap, ran a snake all the way to the grey tank, took off the air valve under the sink and checked it, and the kitchen sink still would not drain. Talked to GD service at a local dealership and they were unsure of the problem over the phone. If the unit does have a 2nd grey tank valve, where is it located? Nothing in the literature I've been able to look at. Thanks folks for any help.
A rig that size with distance between the galley and bath almost certainly has a second grey tank. Maybe look behind the wheels for another handle.
Last edited:
21GFamily
Member
Lol, I had the same exact issue. Once I found that second valve I was back in business.
2020 GMC HD3500 Denali Duramax SRW
2021 Momentum 21G
2020 GMC HD3500 Denali Duramax SRW
2021 Momentum 21G
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