Generally speaking, backing up when flat towing is a bad idea that should be avoided. It is usually best to just avoid getting into situations that require backing up. Failing that, just unhook, back up, re-position, and hook back up.
HOWEVER, contrary to what many will tell you, in many situations it CAN be done as long as you are cautious, go slooow, and use your head.
A lot depends on the specifc toad and its front suspension design, The suspension on some vehicles is much more forgiving than others when being pushed backwards. Jeep Wranglers in particular, even though they are very popular as toads, they seem to be the most unforgiving when it comes to pushing them backwards. Their front wheels will slam to one side almost instantly, and tear something up. I guess, 'tis one of the downsides to a vehicle built so well for off-roading.
OTOH, I've seen some FWD vehicles pushed all the way across a parking lot without a problem. I personally once pushed our Kia minivan a good 30-40' backwards when experimenting in a parking lot. It worked OK as long as I was on a smooth surface and kept the MH wheels straight.
The Motorhome involved plays a role, too. Generally speaking, the more rear overhang the Motorhome has, the more potential 'swing' it could have when backing, making it more difficult to back without jack-knifing the toad.
Personally, I've never yet had to unhook and back up (except of course, at back-in campsites). There were a few times I just 'coasted' backwards (sloooowly) a few feet to clear an obstacle, but other than that, we've always managed to avoid having to back up. However, we've only been doing the flat towing thing for 'bout 3 years. I'm sure one day it'll happen to us; when it does, we'll just deal with it. Good thing about flat towing is that when/if this happens, unhitching and hitching up is so quick and easy, its really not that big a deal.