Best product to seal and preserve the wood under my camper?

Hmm I wonder how well that'll hold up. I'm kinda afraid of this Smith & co stuff, people keep saying it's lethal and potentially damaging to your health but of course it works like a charm.

I had a boat shop in Richmond Ca, about 3 miles from where they were developing their epoxies, like in the mid 1980s and they were great. As I recall the viscosity was mainly as a penetrating sealant, and we used it to coat raw wood as an alternative to fiber glass mixtures.
Very thin, so without mixing with thickeners, it was like milk . Not for any overhead work (dripping into your eyes was not cool). I built a number of marine ply lapstrake boats using it, mainly because at the time it was far cheaper at the time than System 3 epoxy.
All epoxies can be hazardous when activated (2 part mix) but after it has gone off, or starting it's cure, it becomes innocuous and has no nasty after effects (unless you decide to chew on it, and then it's a dental damage problem because it really gets hard).
Turn your trailer upside down and go for it, otherwise just slather on some Tractor Supply fence post paint and rest assured it's water sealed for the rest of your lifetime.
I was not paid to endorse this product nor do I have any interest in it's sales or promotion.
I just like saying that.

POR 15 doesn't work well on wood.

I'm not sure that coating is ideal if your wood is already starting to flake and/or peel. Be sure it is in good condition before proceeding. You might consider a deck product that protects wood, then a coating to protect that from road debris. Any of the rubbery roof products should work. I use Henry's Damp-proofer because it is black and water clean up. Even thick and gooey, it will drip when you least expect it...

Have you confirmed that the wall and roof framing is rot free?