I have never tried to fully seal a trailer.
I "have" sealed large areas, similar to a trailer.

How long must it sit?
Can it be inspected now and then?

Best is in a rather dry location, not sitting outside.
There are products like "DampRid" which are are tubs of a water absorber found at many local places.....grocery stores, hardware stores, big box places like Walmart/Lowes/Home Depot, etc.