How to fix split open wheel well?

Depends if you can get the sides of the split to go back together or in the same plane. If they do, I’d go with what Ralph said, adding Eternabond on the inside.
If not, then either replace the wheelwell, or do something like Guy suggested. And I would glass it inside and out.
I might be tempted to just Eternabond inside and out at first and see how long it lasts. Could be eternally.

If it is just a crack or narrow opening and you can get at it, repair it with fiberglass tape, wetted out with West Systems "G-Flex" two part epoxy.
G-Flex is about the only epoxy that will adhere to anything including plastic.
Follow the directions on the brochure that comes with it.

Guy

Pictures would be good to give you the best advice. On a 12 year old camper it could be many things. Could be rotten flooring, Could be something got thrown up by your tire and damaged the bottom of the trailer. Need to do a little more investigating to see if the flooring inside the cabinet is soft or solid and just punctured.