That's your problem, something is stuck in the blades.
It can be turned in either direction. Sometimes if you turn it back and forth what ever is jammed in there will break. If not it probably something that was accidently dropped into the tank or sink drain when the RV was manufactured,
It's very simple to take the pump out. It just hooks on to the pipe the same way your serwer hose hooks on. Just unclamp the small hose and unplug the wires.
Hold it so inlet is down, tap it, turn screw back & forth and it should fall out.
The screw should always rotate freely if it is unplugged.
since I just purchased this motorhome, I am going to leave it to the rv dealer. thanks.