SoCal Tom
Explorer
The people over on tnttt seem to really like a product called CPES for sealing the outside of their wood trailer builds. Clear Penetrating Epoxy Sealer. I am not sure on the price, but it does sound like it holds up incredibly well. I stumbled upon it when trying to find something to seal the bottom of my pop-up, maybe someone with experience with the stuff will chime in.
I used CPES it's basically epoxy diluted with mineral spirits. I've still seen others that used it have delamination issues. I've wondered myself what mineral spirits does to the glue that holds plywood together. If you decide to use it, don't go overboard.
Tom