My
Scamp carries its spare on the back as do many others. It's balanced by a
battery and LP tank on the tongue. No problem. If you don't have anything on the tongue, that would be a better place for the spare.
You are correct to want heavy stuff over the
axle, but for permanent equipment like the spare, you want it out of the trailer and out of the way.
We carry a large, heavy ice chest and an EZ-Up
awning. They ride on the floor over the
axle, and they're the first things we unload when we set up camp.