I took my PhD in Jabsco marine sanitation last winter. Yes, you can remove the motor without removing the toilet if you have a little room behind it. I had to do it several times and found that it was easier to take out the toilet. Better access to the screws, etc. Mine ran but, in the end, I found that the toilet wasn't the problem. The hoses to the holding tank
had accumulated crud and were blocking the flow. Because the pressure is low, it doesn't take much. I have a Y valve and two discharges, one overboard
and one to the tank. I never did get the overboard to work but managed to get the one to the tank clear. I disconnected the large hose from the Y valve and jammed a piece of rag, about 12 x 4, into the end of a snake. A couple of swipes and all was well.
One more bit of wisdom. After extensive cleaning
and rebuilding, the damned thing stopped working almost immediately. The water
came into the bowl but didn't leave. The moral of the story? Empty the holding tank