If you look on the inside of your back wall, perhaps behind the toilet, you may find what looks like a cover scabbed onto the wall with a couple of small screws. I have a 189FBR and in my case it is right behind the toilet.
Remove the cover and you will see a piece of plywood that is imbedded in the wall or perhaps on the wall's surface. The plywood will have two blind nuts attached to it. These nuts receive the bolts that hold the tire mounting bracket onto the outside wall. It is pretty simple and not a very strong setup.
I have heard that the tire bracket is a notorious spot for water to seep into the wall. I pulled my tire bracket off and found that they did not seal around the bolts at all and moisture could come right in. If enough moisture gets in, it could rot out the plywood and loosen the blind nuts. Worst case, the bracket could fall off, nuts and all leaving a hole in the trailer.
OR, perhaps your tire bracket just vibrated off due to the bolts just loosening up over time due to use. Don't know. You will be able to tell when you pull the cover off.
If you find dry rot in the plywood, replace it with a larger piece and get some new blind nuts and bolts. I think the only reason they used blind nuts is because they mount flush with the surface of the plywood and can be covered more easily. Depending on your skill and on the amount of damage, you may have to resign yourself to the fact that it may look different when you are done. That's OK. The important thing is to make it strong.
When you dig into it, post some pictures of what's going on.
It is a good idea to inspect the spare tire bracket often....its kind of cheesy.
2008 - 189 FBR
2007 Jeep WK Limited CRD