Nice job, I think you are good to go. I had Nexus install a custom {welded}, mount under the rear of my 24' Class C when we were passing through Elkhart. This is where you want your spare. The weight is down low and forward of the rear of the coach. Mounting on the rear bumper is a disaster waiting to happen.... and yes, I know this first hand.
I bought a rear bumper mount and after hitting just about every pothole between Tucson and Maine its finally fractured leaving the spare hanging at a 60 degree angle out from the bumper. Lots of Class C's have such wimpy bumpers they cannot support the weight and you are very likely to lose the entire bumper. :S
As for the folks that think they don't need a spare.... good luck with that.
:C
