I just pulled that side panel off to repair the black tank flusher spray head. The black tank sits on top of the fresh water tank and the fresh water sensor was very visible once that panel is off. There were 4 screws visible on the forward part of that panel, about 4 inches from the top and 4 inches from the bottom, same on the other end of that panel.They are in that tiny space between the two side panels to the rear. I'm talking about the panel just forward of the rear wheels on the passenger side of my 40 AP motorhome. The panel is not that heavy and is a one person job to remove and reinstall.
Once the panel is loose, you can remove the clamp holding the fresh water fill hose and the panel will come free.
2016 Allegro Bus 40 AP
2015 Cadillac SRX towed car
1991 Avanti Convertible/2019 Tesla Model X