The shower walls in RVs are typically very thin and flex easily so attaching something just to the shower wall is out.
You MUST anchor a grab bar to something solid in the wall like studs. As said above, you'd need to attach to an inside wall. May be possible but could involve a lot of work. Even if you could find studs, there may not be enough "meat" on them to put in the screws that are required for them. You could have 1x2 (not good) or 2x3 (good) in the walls.
Maybe if you posted a link to the layout in your camper it may help for suggestions.
They do make "grab poles" for use in a shower or bathtub. Google "shower grab pole" or "shower security pole". This might be an option.