IMO the bumper on a 17z is cosmetic, and I wouldn't consider mounting anything "heavy" even if it was reinforced. I'm looking for a tire carrier I can bolt into the frame. That way if I don't like it, it's easily removed. ... will probably have to modify a stock product to do so. I plan on consulting with a few experienced towing businesses before doing anything.
Jayco and their dealers might not be able to discuss modifications as freely as they would like to satisfy their customers. I understand and accept that. If we were "in their place" we would probably say (or not say) the same thing(s). All that said, my Z is out of warranty in a week or two, and If I get an experienced opinion that I can safely mount a spare tire on the rear, I'm going to do it.
On the lighter side: Murphy's Law is in full force here. If you bury your spare under a full load in the back of your truck, you will have a flat. If you spend the time and money to have it conveniently located but out of the way, you will never need it.
... if the cabinet below the rear bed could be enlarged to carry the spare tire.
I don't think it will fit there, but that would be an excellent place to haul it. ... maybe a small floor bracket to keep it from moving with a means to secure it to the back wall. But that might block the entrance. ... on to plans C, D, and E.