Hi list,
I'm going to butt in here and make a few statements. Right after the shore
water enters the trailer, there are likely two valves that might be
recognized. One is a pressure valve and the other is a backflow valve.
These may be combined into one valve. Up front with the potable water tank
is a backflow valve. The back flow valves work on pressure, not a manual
handle.
These valves can be damaged or inoperative. Especially if the potable water
tank were not used for many years. My '69 Safari had an ungodly number of
valves up front to dump the potable water tank and water lines. Same thing
in the rear. If you are dealing with lots of valves, label them as you find
out what they control.
Potable water should flow through the pump, through a backflow preventer,
and then into the water system. The backflow at the shore line prevents
water from exiting the fitting. With the shoreline connected and the pump
off, the water enters the system and is prevented from flowing back through
the pump and into the potable water tank by the other back flow valve.
A simple system and effective. It would be a disaster if you could fill the
potable water tank from the shore connection unless the plumbing system were
modified with manual shut offs.
If you attempt to pump water from the potable tank and the pump drain or
tank drain lines are open, there wouldn't be enough pressure to open the
backflow valve. Also, if the pump isn't really putting out pressurized
water, there might not be enough pressure to open the valve. Be sure the
pump is really working at specification. The pump could run great, but
flapper valves or ball valves might not be operative - sort of sucking wind.
Most likely, you need a new pump.
Hope this helps,
'69 Safari, Joy