Notice today while cleaning that I was getting a huge bubble back from the toilet when flushing and man the stink!! I was convinced that the vent was plugged so I went to drain the tank so that I could use some air to open up the vent. I realized that I hadn't emptied the tank from the last trip, I usually do but sometimes if the trips are close, I drain and clean just before departure. I check the vent pipe and now it was open--hmmmm-- I filled the tank with a 5 gallon bucket and when I got 12 gallons or so in the tank, I started to get so back feed of air from the toilet. Flushed repeatedly and the same thing, each time I got to about 12 gallons in the tank, if I flushed the toilet, I would get a big burp of air from the main toilet valve.

Long story short, the vent pipe is sticking down into the tank so that when 12 gallons of waste or water is present, the vent pipe is covered and will not vent, furthermore, the drain pipe from the toilet goes straight down and also is to long. the tank will refuse any more water after 18 gallons, the tank is stamped 25 gallons.

Anyway, after the waste water rises to cover the vent the only place that the air can go is out thru the toilet when flushed.

More work ahead, I will probably increase the size of the tank when I fix the vent pipe length because this is the tank that has a bad crack in it that I fixed right after purchase Feb 2006

I had a situation similar to that on a Fleetwood product. The vent tube stuck too far down into the tank. This canceled its ability to vent gasses when the tank contents rose to its lower end.

The answer, of course, was to raise it. But how much? I had a great dread of pulling it up too far and really complicating things. So I drained and washed the tank, filled to the top with fresh water, and let out an inch's worth, as measured at the toilet chute. I then pulled the vent tube up a half-inch at a time until it no longer bubbled when I blew down into it.

I used a Saran Wrap gasket when I blew, but those who are disinclined to put their mouth on the vent and blow can use a heat gun (on the cold air setting) and measure the pressure with a manometer. The pressure will drop dramatically when the vent tube is raised above the water level.

Then I secured the vent tube in the proper location, knowing that the vent was a high as possible.

The next one I did, a couple of RVs later, I just used a hook to raise the pipe and stopped when the hook bottomed out on the tank top. Much more practical.

Problem I have it the vent tube shares the duty of the sink drain input to the black tank and there is a tee right there as it goes into the tank. The top of the tee is right up against the floor that the toilet sits on the tee (and vent pipe) will not budge, up or down.

I will have to remove the tank to fix unfortunately but, as I said I will probably install a larger tank while I am at it.