About parging the entire exterior shell insides... My suggestion is spot application of Vulkem where needed on rivet backs and seam lines but do not coat everything - instead work on every outside seam and gasket until there is no/low chance of water intrusion.
With the rear floor out of my '73, I placed a jack & prop under the rear utility hatch and tensioned the heck out of everything to lift the shell off the frame just 1/2-inch...
Then I noticed running water, not just drips but a slender stream that poured out for nearly 30 seconds, with drips continuing for quite a while. It seems that the inner seams and rivets slathered with Vulkem for 35 years had freeze cycles make voids and such into bladders with a considerable volume along those seams that compressing the structure with the hydraulic jack allowed to dribble out.
Looking close I see 4 or 5 places that the caulk bond is a clean separation from the aluminum (10" below taillights, sheet overlaps above rear utility hatch, above battery locker) which channels any wicked water straight into floor channel cup... That, and the existence of bladder reservoirs, means long term exterior gaps needed attention years ago.... So I now hate the idea of capturing and storing water behind the shell with coatings of any type.
With the belly pan - ensure all the line drains perforate the skins to drop onto the ground direct; my trailers might have had ALOT less damage if that were positively accomplished instead of direct onto skins to leak through seams or a plastic drain hose that kinks and retreats up over time.
And providing air vents throughout structure - if they were a good thing Airstream would have delivered them that way. You could be feeding the damp with condensation cycles unknowingly, especially using chimney effect from beneath flooring up - the water vapor at ground level goes 100% twice a day here w/ morning and evening dews, draw that heavy damp air into a trailer is simply adding it's exposure levels... Relentless vigilance on exterior caulks and seams is the way to proceded.
Also - to keep rain from wicking into belly skin sheet seams I plan on making a drip-edge to keep dew & rain from capillary wicking upwards, attached is a photo..