Fresh water use

He said that the pump doesn't run and there is no leaks. Perhaps he is arriving with very little water because it was lost out the vents while traveling. BTDT

Are you 100% sure you have your water pumps turned off? If not, that's where the water is going. I like it that way as I get better pressure when the pumps are on but it will drain the tank over time.

Sounds like you are leaving the pump on when hooked to city water which won't hurt the pump but if you use a pressure regulator that is less than the pump pressure it will use the tank water first. I have an old Jayco designer and the pump is very quiet.

One thing I just thought about, on our Jayco we have a single hose hook up and a 5 valve water control panel. If memory serves, one of the valves selects between the tank and city water. If you have the same valve set up, is the select valve set half way between the two?

Edit: I went back and read your 1st post and is sounds like you have a similar control panel to our camper. The valve in question selects between "city" and "normal" which should be the tank. If it is set to city it should not pull water from the tank even if the pump is left on.

Thanks for the ideas. There is no visible water leak under the rig. (None above it, either, in case you’re wondering!) The pump runs normally when no water source is hooked up, and does not run when the city water position is chosen. I usually use a regulator, but I thought that was mainly to guard against too high city pressures.
David

Your condition is not normal. So "No" is the answer to your question.

IF you use a water regulator the pump may kick in to supplement
In some parks again the pressure in the city line may be less than the pump pressure

You may also have a leak

This is in addition to the above comments Turn pump off when hooked to city and see what happens,

Did you turn the pump switch off? If the city water pressure is less than the "on" pressure for the pump, the pump will run and deplete your onboard tank. Do you hear the pump running when you open a faucet?

Valves are in the wrong position. Check owners manual. Label in compartment should show correct valve positon for city water usage.

If I understand your post, the NO it should not do that. And I don't understand how it is happening unless the pump is on, and is running as you use water. Or you have a leak.