The underside of your camper is perfectly well ventilated. That means it dries quickly if it gets wet.
It's also treated from the factory. It may make more sense to inspect it about once a year...say when you lube things. As with the underside of an outdoor deck, you'd want to watch for any any evidence of mold or fungus. If you see anything suspicious, a deck wash product (with either bleach or oxy bleach) will kill whatever it is.
But my 2014 PUP shows no signs of such deterioration. And I think that sealing the underside might do more harm than good.
Again, just a hunch.
Jim & Renee
2020 Jayco Jay Feather X-213
previously 2014 Forest River/Rockwood HW 277
2006 Ram 1500 4WD Crew with Firestone Airbags
Every weekend boondocking in the National Forests or at Lake Wellington