Having towed a car 4 wheels down for over 20 years, I agree with all the previous posters - - not a good idea to back up with the toad attached. However, there have been a few times when I did back up with the toad attached, generally no more than 2 or 3 feet but at least a couple of times as much as 6 or 7 feet - - and never experienced any problems.
I believe that the key is to back up the same way you drove forward into whatever situation you find yourselves. In other words, never turn the steering wheel in the motorhome to attempt to make the toad go anywhere other than where it has just been. If you follow that advice you will put the minimum amount of pressure on the tow bar.