My class B has a full size spare tire too, mounted on one of the back doors. When I purchased it, all the tires were about 9 years old. I immediately replaced the 4 that were on the vehicle, but left the existing spare on the back. Now I'm wondering at what point I should purchase a new replacement for the spare. Sometime this year, that 9 year old spare will turn into a 10 year old spare. I'm thinking that even for a spare tire, 10 years is too old. I should be able to get a replacement for ~$150. If ever caught by a flat in a remote area with no cell reception, I'll be happy for a good spare. That $150 will, at that point, feel like money well spent.
The rule of thumb is that tires should be replaced by the time they are six years old, regardless of wear.
Tires Expire in Six Years - Tire Safety Group