That city water inlet check valve is a cheap plastic plunger/spring/o-ring assembly.
Even though it moves in/out freely doesn't mean that the o-ring is in proper position..they can get dislodged and block/restrict flow.
If you have good water flow thru hose with and without the regulator...then it's that cheap plastic check valve causing your low flow problem.
You can remove the check valve internals to get good flow back. then replace city water inlet assembly later OR use a 3/4" pvc plug to cap it off.
The check valve is just there to 'seal' city water inlet when using on-board pump.
I installed a 90* elbow with a garden hose shut off valve on mine years ago.
Hook up hose and open valve when I use city water or leave valve closed when I use on-board pump
Have same set up on black tank flush connection.

