My wife has bad knees. We have a 5th wheel with 4 steps.
I added a Burr Handi Lift http://www.burractuators.com/lifts/handy_lift.htm
To help her get inside.
She uses a mobility scooter to get around. Most scooters break down into several parts to make it easy to transport. Hers breaks down into six parts with each battery being the heaviest pieces. I carry it in the back of the truck. Takes less than 10 minutes to put together or take apart.
Another option is a Hoyer Patient Lift. We used one of these to get my wheelchair bound dad in and out of our truck when he traveled with us. The Handi-Lift was used to get him and his wheelchair inside of the 5th wheel.
http://www.spinlife.com/Bestcare-Lifts-Genesis-400-Hydraulic-Manual-Patient-Lift/spec.cfm?productID=77888
We found the mobility scooter and hoyer lift on Craigslist for 1/4 the new price. Both were in new condition.