well...first of all, with the "stock" configuration, you can't fill the water tank with the city water connection. You fill it through the water filler spout.
with the city water connected and all the valves closed, the city water pressure will simply pressurize the pipes. (there's a check-valve that prevents water from getting into the tank; the pump can pressurize the lines from the other end, but water can't flow back toward the tank. there's a check-valve on the other end of the system, as well, just before the city water inlet, that keeps pumped water from going out there). If water is coming out the back drain, either the valve is actually open, or the pressure relief valve is stuck open...or not "stuck" and doing its job of relieving excessive line pressure...which would mean that your on-board regulator isn't doing its job.
you'll have to figure out which drain tube its coming out of; look under the back, and you should see 2. Then go look under the bathroom sink and see if you can deterimine if its the drain valve, or relief.
'73 Safari 23'
'00 Dodge Ram 1500 4x4 QC