A micro switch on the shift mechanism sometimes confuses reverse and park and will not allow you to back up because it thinks you went into park.
The switch has to be adjusted perfectly but from my experience when you go to back up and the body of your motorhome gets twisted a little bit it seems to somehow or another mess up that switch. The switch is high up above the left front wheel and can be lubricated with some grease if you can reach up there far enough with something.
I have had problems with it but I don't think it is a common problem because I don't remember many other posts about it.