Water Inlet Restriction or Check valve?

[NitroStar]

I have a 2015 Flagstaff. I don't have much water flow at any faucet. I'm new to campers though. When I flush the toilet, I hear a squeaking sound coming from under the sink sometimes. This is where the water inlet, pump, and filter are located. I borrowed a large 3/4" chicken house regulator and it made no difference to my water flow when taking a shower. Guy that loaned me the regulator told me I had a check valve or other restriction if it didn't make a difference. Any tips or comments?

[Rivnut]

I have read earlier posts that there is a check valve/anti siphon valve in line from city water. It happened to me after disconnecting city water and using the pump and back to city water. I believe it is a problem with the check valve sticking. It needs to be bled to break the pressure. Search for check valve, antiphon valve in the form and you will find several cures.
regards
rivnut