Before having a knee replacement find out what type of knee they are going to put in and then get yourself tested for any possible allergic reaction. While rare if you are the lucky guy with a reaction you are seriously screwed.... Been there done that.
My first replacement was Chromium knee and it turns out I am deathly allergic to Cobalt and Nickel both of which are in the Chromium. My body was rejecting that knee before I was off of the operating table. After 17 days in the hospital followed by 6 months of excruciating physical therapy and only getting worse I finally fired my surgeon and got a second opinion. The new surgeon took one look at my swollen, warm to the touch knee and stated un equivocally "That has to come out.... and soon".
The second surgery, only 14 days in the hospital this time, called a revision {doctor speak for we screwed up the first one}, went better but the double layer of scar tissue will always be an issue. I can do stuff now but everything I do hurts, 24/7. For the record I was not/am not overweight.
Getting back to the RV side of the question I use an 8" high single step at the bottom of the two interior entry steps on our Class C and it helped tremendously both for me and our recently departed doggie. Good luck!
:C