I am no plumber.... and do not know about your shower knob... but we have a Thetford toilet in our 1963
Bambi. Very close to the wall. My husband disassembled the entire trailer... but once he was putting the toilet back in... I got elected to turn the tiny wrench 1/4 of a turn over and over again because I have much smaller hands and wrists and could just (barely) get in the tight spot to do it! Got any kids? They might fit....
Mrs. NorCal Bambi (traveling in S Tardis)