We also suffered the poor flow from our kitchen faucet. It started with the cold side and then the hot side flow rate also declined. After replacing the faucet with a like $400 + replacement, I asked the service guy to save me all the parts.
After checking, I found the lines have a screen filter installed where the coach lines connect and they were plugged from trash that was left in the holding tank.
Took 3 or 4 years to get enough to stop up but flow was almost zero. Now we have a replacement faucet assy. if ever needed in the stick & brick or our coach.