I agree with the others that you'll need to test them out.
Also remember that with chronic ailments, you may have some nights when you hurt more than others. So, even if you find you can get in and out comfortably most of the time, you'll need a bed option for those times you can't.
A short class C may be a good option. Mine is 21ft and I use the cabover bed most all the time. However, if I injured myself or such, I could always sleep on the couch below. It gives me an option that a TC couldn't necessarily provide (without using the dinette as that's where the bird's cage is).