Are you working with a motor home, or trailer (Argosy 26 came both ways)? If you are working with a trailer, I would advise against putting the spare back there. If you are working with a motor home, many people have done this, and most of the somewhat newer Airstream motor homes had a fiberglass spare cover molded into the back over the bumper.
You should be able to buy a spare tire bracket from an RV dealer, if you can't find one that is specific to your coach, and generic fiberglass covers are available, some even have provisions for a small padlock to discourage "borrowing". You would just need to paint it in a color approximating your motor home.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.