Please do yourself a service and search the forum for 'bumpers' or 'bike racks'.
There have been many threads/posts regarding this issue. These bumpers are very poorly designed and not very strong. In our travels, we have seen several trailers on the side of the road with the owners trying to tie up a failed bumper mounted bike rack. Keystone rates their bumpers at 200#. Figure the weight of the spare wheel, plus the weight of the rack & bikes - then figure in the dynamic loading caused by the weight. I wouldn't do it without reinforcing the bumper. JMHO