We have a somewhat poky water pump too on our (new to us) '77 Sovereign
. I wonder if ours has an intake filter like yours that we could check. Where would we look to determine this?
I have the FlowJet type pump and it has a screw on plastic filter on the intake side of the water pump.. I just unhooked it and took it outside and blew backwards into it to get the main slim plug out,, then washed with some water to back flush it some more..
The PO,, did not care for this trailer like he should have.. The water heater was full of water,, and a very poor job of winterizing all around.. I'm sure he left 10 gallons of water in the tank for years and it started to grow things inside..
Once home I added some clorox ,, and moved the trailer back and forth to slosh it around in the tank.. Then drained and flush 3 more times.. I had to do this trick on our SOB trailer when we first got it home..
Another area to look for is any small air leaks in the suction side of the pump.. Most times if its a air leak the pump will not shut off once full pressure is reached.
We use too use the larger FlowJets,,, to add fertilizer in our crop rows as we planted.. Some would last for years and others just a season..
Another option to look at is to make sure the pump has a good ground.. A jumper wire from a known ground to the black wire on the pump will let you know.. If the pump motor speeds up with the added ground wire you found your problem.
Sodbust,, NW Kansas