Originally Posted by ependydad
Sounds like the vent tube is either clogged or dropped down into the tank.
I’m not exactly sure how you check/fix that, but that’s possibly the issue.
What you describe is a vent problem. You verified this with your experiment, opening the drain valve.
It is close to impossible for a vent to just get clogged, but if, like ependydad suggested, the tank vent is pushed too far down into the tank, it will become “clogged” with water and, perhaps, other stuff that may have gotten into it because of it being too far into the tank. The root problem, though, is probably the vent being too far down into the tank.
What I am going to describe is assuming the vent connection at the tank is with a rubber doughnut/slip-joint seal. If it is a glued joint, it will have a stop that makes it virtually impossible for the vent to be pushed too far into the tank, but having this type of connection is unlikely.
You (or a tech) can determine how far the vent is dropped into the tank from the roof, but only if the vent goes straight (not horizontally tied-into another vent) up from the tank and through the roof — hopefully you have independent gray vent and black vent pipes through the roof. You will first need to remove the vent cap from the pipe on the roof. Drop the dumb end of a tape measure down the vent until it stops. Record this measurement. Now, slowly pull the tape back up until it hooks onto the bottom of the vent pipe inside of the tank. Record this measurement, then subtract this measurement from your first measurement. This will tell you how much distance there is from the bottom of your vent pipe to the bottom of the tank. Now you need to work out the depth of your tank at the point where the vent penetrates the top of it — you will have to open up whatever you must to access your tank and measure it. The vent should protrude into the tank as little as possible, giving consideration for movement of the tank, so the vent doesn’t come out of its seal during normal operation. If the tank is, say, 12” deep at the penetration, I think about 1-1/2” to 2” penetration would be good. If you need to raise the vent, mark the pipe so you know how much you are raising it. Try slowly turning the pipe, while at the same time slowly pulling it up. Be careful, because you don’t want to pull the gasket out of the tank and you don’t want the pipe to come all of the way out of the gasket. Once you have the vent where you want it, cut it off and replace the cap.
I am a plumber, so I would be comfortable doing this, but I still wouldn’t do it if my RV was as new as yours. I would still do the measuring part, though, to verify the problem and present my findings to my dealer. Then I would have my dealer fix it. I can only imagine the back-and-forth with a dealer trying this and trying that at the expense of your lost time, only to finally repair it after you have lost a whole season of camping due to your RV sitting at the dealer.