The first season we had the 342 I had to replace the shower sump. I ran into a problem with the new one the next season where it would run but not pump. The solution was to open the box, pull out the pump and put it back into place. Then it pumped great. I noticed the problem again on our long trip. It seemed the worst after a bumpy ride.
I'm wondering if water is getting into the discharge line and causing some kind of hydro-lock. The only fix that comes to mind is a check valve. Guess what I need is a good plumber. Any thoughts?