Easy way to fix this is to put a check valve in front of the water pump...
I had to do this on my Prowler earlier this year and solved the problem!
I opted for the Camco brass check valve instead of the plastic cheapies from other sources.
Under $10 at Camping World, although I had to pay the premium $13 in Elephant Butte, NM
Took less than five minutes and no tools to install!