Hello, Owl!
Five years in my fiver and I had to replace the water pressure regulator, the one you hook to the hose outside, at least that many times. And the water filter (mine was built in, not the kind you add to the hose). And clean all the faucet screens, also the screen at the city water connection where the hose connects. Assuming you have done all of that, ditto what HDR said about check valves. Never could locate mine because doing all the other stuff fixed low flow problems.
Once, in the Class B which I bought used, I aggressively cleaned the fresh water holding tank. It broke loose all kinds of gunk and when I turned on the water inside, it blocked the water lines. I know you move around in your fiver, maybe something like that has happened?
Does the flow slow down when you use water from the tank with the pump? Or just when hooked to city water?
Best wishes, Friend!