Will you be able to climb the steps to get into your fifth wheel? If not the entry door may have to be widened because lifts need extra room even when used with narrow wheelchairs. This site has a lot of different types of lifts -
http://www.startrackslifts.com/ Another option might be to buy a used RV that has been modified -
http://www.disableddealer.com/item_search.asp?category=1020 We fulltime in a small class C that has been built to make it accessible for me(Karen). It has a Super Arm lift, a wider door, the dinette is cut down to half and the bathroom door and a wall section has been replaced with a folding door. With just those few alterations I have access to everything in the motorhome and room to spin in a complete circle at either end.
Please let me know if you have any more questions.