Daft question, but why do you wish to put the bike on the back, and not where it is designed to go. Unless it is far too big to fit the garage.
The only thing you will have to check is that the body and chassis rails are sufficient strength to take the load as you do not wish it to 'fall off' the back. Although the garage is designed to take the weight, it is possible that the rigidity of the body around it is helping to allow this without something too unwieldy. Check with the maker of the motorhome , and of course the insurers, that this is ok and then get a specialist Company to do the work.