I confess I could only imagine myself scraping the under-mount spare on something, which makes the bumper mount seem a lot safer. The $140 vs $40 difference didn't make ether seem that painful to me.
I know my class A is heavier than a Class C, but you can make a good case for making sure your motorhome policy covers roadside service. There's a reason they don't come with a spare, let alone a jack and tire wrench.
2005 Winnebago Sightseer 29R Class A
8.1 Workhorse - GM 4 speed