My water pump on my 2008 Starcraft 14 R/T POPUP trailer has a valve with a short hose between the pump and the fresh water tank. This short hose is used to stick down inside a bottle of the pink stuff when winterizing the lines.
If your setup is like this you may have the valve in the wrong position and your pump is pulling against the closed off short hose.. My short hose has a cap on its end..
This setup on mine is also a great way to test my water pump as I can stick this short hose down into a bucket of water and the pump then will pick that up just fine showing the pump is working...
my setup looks like this google photo...
My POPUP Trailer water flow is similar to this water flow diagram... My unit only has a Fresh water tank...
google image
Roy ken