There are usually more than one check valve in RV's so to narrow it down to your specific problem search "city water filling fresh water tank".
Or you can do the following
Turn off the city water, open a faucet, turn on your water pump and let it run a bit, turn off faucet, let water pump run til it shuts off, open faucet and let it run, your wp will turn on again, turn off faucet, let wp run until it stops then turn off wp, turn on city water. You may or may not have to follow the above a second time.
Never run your water pump while on city water.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989, seasonal since 2000
Car Shredder Op/Tech, Scrap Metal Recycling - retired
The Faster I Go, The Behinder I Get.