Oh that water thing again!
I posted several weeks ago problems with my water system. No h2o from the city pressure and the pump provided no water as well!
I have the same trailer as you do, '78 31' International, rear twin and center bath. I have also solved my problems and some of this might be of help to you. What is in my system is first, the city water attachment point. Second, is the one-way check valve to prevent pumped water from exiting the rear of the trailer! Next is the Watts pressure regulator (adjustable, came from the factory set at 45psi (mine did anyway)), and finally, an over pressure pop off valve (Watts brand as well). My overpressure valve decal has disintergrated so I can't tell you the pop off pressure. The PO of my trailer never has a screen at the city water entrance or any plug to seal the system. Bugs started to make some sort of nest there and when city water was hooked up the water pushed the nesting material into the interior plumbing. This material eventually was stopped by the one-way valve, eventually causing the city water system to be inoperative.
The PO also seemed to have been using the internal tank and pump to provide his water. When I tried the system pump it turned on and ran ... and ran ... and ran. No H2O to the system. Discovered the water filter/strainer for the water entering the pump was cracked causing the pump to run on as it couldn't develop a vacuum to draw the water. Fixed that and discovered the pump worked fine except for the cracked bottom cover and side case, both of which leaked severly. This was probably due to poor winterization the year prior to my purchase. This was an estate sale so couldn't talk to PO!
I have cleaned up all of the valves, checked the pressure valve and added a whole house filter to the city water system. I have also replaced the original pump with a FloJet VSD pump. It's quiet and provides copious amounts of water (3.7 gpm). This pump requires no accumulator tank and, did I say, it is quiet!!!
The bottom line is with the industrial quality pressure regulator, proper filtration and high quality one way checkvalve the city water system is near bullet proof. Bring on that high pressure park flow!!!
Hope this is of some help,
Old trucks and old trailers . . . a comfortable combination!
A-6E All Weather Attack Driver, BUFF
USN Aircraft Maintenance Officer, Ret.
'91 Suburban R-2500 w/BBC
'78 Honda GL-1000
'72 Triumph T-100R Daytona