I've been around the UP of Michigan enough to see that none mount them on the tailgate, unless it's an SUV (Bronco, Scout, Blazers, etc.). Most that have them are mounted in the box towards the front. I haven't really seen any mounted on the plow frame, but I have seen ones mounted on non-plow trucks on a special bracket on the front, generally ones that have a camper mounted in the box. I have also seen a few that have no spare at all!
Biggest reason I think that they aren't under the bed is because the lifting/lowering mechanism has rusted out/up.