Is there a reason she cannot use an outdoor bicycle?  There are folder
bikes that could be stowed in a compartment of an RV.

Not a bike, but I did find a cheap ($79 USD, IIRC) stair-stepper that
folds pretty easily to a fairly compact size.   It could be set up in
'most any but the smallest RV's, I think.   It's a little heavy, but is
framed in round tubing, so you can grab it securely anywhere.  I bought
it for home use, tho' I could take it along in my B van, bungeed in the
aisle.  

I bought it new locally from Play It Again Sports.  The brand is
'InMotion'.  I'll look for a model number if you need it.  

Jim, "Are we there yet?"

Jump rope.