Coach-man wrote:
I at first thought of the AAV, but after reading your reply I have to go with your vent being blocked. I have read that in manufacture, they can sometimes place the vent way to low in the tank, and when you start filling the tank with liquids, it blocks the bottom of the vent pipe in the tank. Run a hose down the vent to see if something was blocking it, if that does not work, then it must be blocked at the bottom.
Or the vent detached and has fallen down. That's a common one, too, given the "normal" construction of RVs these days.
Lyle