Best place to access the water pump on the 3905 is the from the driver side. Open the door that gives you access to you plumbing controls, fresh water, dump handles etc.
Remove the panel just to the left of the control panel. You might have to crawl in the the other side to remove some of the aluminum angle holding it in place.
The pump will be on the floor right behind that panel.
It is easiest to remove the hose not from the pump but where the hose connect to the plumbing. The hoses will only be a couple of feet long.
Pump should be screwed to the floor with four screws. Not easy to see but do able with light and a cordless drill with extension or just a screw drive.
Note the wiring. I forget the coding on the stuff. Black to black and red to white. Can't recall right now. Just make a note.
I had an Mobile RV Tech replace my pump once. He did it from the passengers side, laying on this belly and reaching in deep in to get to it. He was wrong. It is much easier from the driver side.
I replaced it again myself after the second pump failed.
Be prepared to possible clean up a bit of a mess you may find it there.