Not knee replacement, but 2 years ago I blew the cartilage out on my right knee and had surgery. Not sure which was worse, the original blow-out or the surgery. Sometimes I wish I would have skipped the surgery, I might have less pain now (2 years later). Meanwhile, the knee, actually, the entire leg hurts and throbs. The pain moves to different places, but always hurts, always walk with a limp, and on pain killers now.
Fortunate, the other leg is good, so the good leg does all the lift, one step at a time. Makes me VERY happy I have a travel trailer and not a 5er. 5er's have steps to the second level. From the ground to the first floor, can have 3 to 5 steps. From the bottom level to the top level can be 3 to 5 steps. That's just way to many steps.
So, really, if you have bad knees, bad legs, walking problems, you want to eliminate stairs and steps as much as possible. You choose which type of RV suits you.
This last summer was camping in Missouri. Next camper over from us had an older guy using 2 canes. Poor old guy could barely stand. I watched him come and go from the camper (travel trailer). He would sit down on the floor, dangle his feet out the door with his feet on the ground and then stand up. (no steps at all.) That's how he got back inside too. He would stand at the door backwards and simply sit on the floor, skoot himself inside and then figure out how to stand (I suppose). By then, he had the door shut.
Bottom line, if there is a will, there is a way.