Yup that is where I would start: pop the taillights off the coach and check. You'll likely find that water got back there and there is a bit of rust and maybe even some wood rot as there is no place for the water to go once it gets back there. The gasket on those lights is just a cheap foam one that, when compressed, doesn't provide a very good seal.
I did the following for my taillights:
- Coated the inside of the recess with flex seal to prevent rot from any water intrusion
- Replaced the entire fixtures with an LED one
- Replaced the foam gasket with a rubber one (just picked up some rubber from the home improvement store and cut it to size using the old one as a template)