Ok, after just driving my RV for 3 hours without the cruise yesterday, I have one more suggestion. If you can prop something up between the seat and the arm rest, that gives your leg something to rest against. What I discovered is that while my motorhome seats are very comfortable, they are also soft, which means there is nothing keeping my leg/knee from naturally falling wider than the pedal location. This means my lower leg twists on a funny angle and puts a strain on my upper leg and lower back holding my leg in. If the seat had a more prominent bucket shape, that would help, but I think if you come up with some side support, that would solve it. You could probably take a towel and wrap it a couple of times around the armrest and then drape it down to the seat so you are sitting on the other end. This would hold your leg in, so it does not strain your back. Assuming that solves it, then figure out something more creative and permanent.