Originally Posted by bubbles
Might check that the overflow tubing is not blocked (it's also the vent for the tank) and the inline filter to the pump if on the input side.
X2 also do you have an in Trailer water filter to check? Also depending on your pump you might have a screen that could be clogged but the pump itself. Another thing that use to happen to my old TT is an air lock. The same situation as you decribed use to happen. I had to play with opening and close faucets, flushing the toilet, ect to get rid of it. If all else fails a new pump is around $80 and easy yo replace.