Some RV parks have a wheelchair accessible shower. Many just have the home-type stall showers, but they often have a floor drain in the private dressing area outside each stall. You could simply unscrew the showerhead and screw on a handheld shower that you bring with you, and shower her in the dressing area.
Or you could make a simple ramp, that you also carry with you, to get her chair over the lip of the shower stall. But those stalls are small!
You will also want to make a sign to hang on the outside door, warning women that there is a man in the women's bathroom :).
I think it's great that you are helping make this happen!