chment]Bonnie, i just removed a plywood box from the back of my 13' Boler
and mounted the spare tire holder directly onto the bumper. I have seen some mounted directly thru the shell, but it is a 1970 and i didn't want to stress the fiberglass shell if at all possible.
Was pretty straightforward to mount it, although i did have to get some custom u-bolts made in order to fit the bumper as it is pretty narrow. i got the ubolts done at the local spring manufacturing shop -- they install leaf springs, do lots of frame repairs etc.,
i have attached 3 pics, one of the "naked" bumper, a side view of the spare tire mount and the rear view. the mount was recycled, but is a pretty standard after-market model. came with a couple of bolt-on brackets.