Here are some threads about the fresh water tank filling when hooked to city water.
http://www.thorforums.com/forums/f3/...ump-16128.html
http://www.thorforums.com/forums/f8/...ows-14803.html
http://www.thorforums.com/forums/f8/...psi-15496.html
Lots of threads if you use the advanced search.
We've never had this problem with any rv, but we
always turn the pump off before turning the city water on.
Try this
Turn the city off.
Turn the pump off on off on OFF
then open a faucet to releave pressure then close the faucet.
Make sure the pump is off then turn the city water on.
Check the water level for the water tank, often.