Here are the steps I would take.
Is there water pressure at the campground faucet?
Is there water pressure at the other end of the hose (including pressure regulator is used)?
Using the water pump on your rig, do you have pressure inside the coach?
That should narrow now the issue.
If the answer is yes to all three questions, a full-coach water filter, if you have one, is a candidate for causing the problem, especially since you had a dropping in pressure over a period of time. Another candidate would be the the check valve in the city water connector.
If you still have the problem, let us know the answer to the three questions, above, and we can troubleshoot from there.
Tom
2015 Meridian 36M
2006 CR-V toad
3 golden retrievers (Breeze, Jinks, Razz)
1 border collie (Boogie)