I was thinking of doing the same thing until I saw a failure of one. It was a factory square tubed bumper on an older Jayco TT. The owner had the factory spare tire mounted to the bumper as well as an add-on bike rack mounted to the bumper. He was using a 2" bolt-on receiver type hitch that attaches directly to his bumper with no reinforcing. He had 4 bicycles on the rack. The bumper failed on the freeway, the bumper gave way right in the center where the bikes were mounted (the bumper twisted). All of the bicycles where damaged and one came dislodged and hit a car on the freeway. Luckily no one was hurt. Soon after that I have a friend that had the same thing happen on his motor home. If I were to do it I would get an actual receiver hitch made for a TT.