Yikes! There are three problems here -- two are fairly easy to solve. One is not.
Let's start with the worst news. It sounds like you have a leak that caused the wall to rot. Chances are good that it was around the vent for the black tank, based on where it occurred. You have to find that leak and patch it. Eternabond and Dicor will be your best friends.
Now that the leak is fixed, patch the back wall. A surfboard repair shop can help you with replacing the fiberglass. It will never look like new, but that's ok. (If there is no surfboard shop near you, I bet there is a boat repair facility.) Make a plywood sandwich: fiberglass outside, plywood in the middle, fiberglass inside. Strong.
Finally, remount the spare tire. Use wide steel plates inside the wall to support the weight. Here is a description of how I approached the same job:
The LMIC (Look Mom I'm Camping): Gas can mount on top of spare tire
Good luck, and ask lots of questions! This is a big hassle, but it is not the end of the world.