We had a Coleman popup for 13 years. Drove in all kinds of rain storms, camped in rain, and always stored it outside. When we sold it, the particle board underneath the trailer looked as good as the day we bought it. I didn't see any obvious signs of sealer or coating from the factory, but it did amaze me that it was built with exposed particle board, and really amazed me that it held up so well.
Seems like if your trailer is solid now, it should stay that way. You could try some polyurethane or some other wood sealant in a small spot and see how it takes. Certainly wouldn't be a huge job to seal that wood if you like how it goes on.
2005 F-150 FX4
2013 Rockwood Roo 23 IKSS