First, Go to harbor freight and get this:
https://www.harborfreight.com/search?q=moisture%20meter
Second, open up a baggage door (that communicates with the camper's interior) and cut to fit a cardboard insert with a cutout in the center and duct tape into place...Position a powerful leaf blower (or two) on a ladder and insert nozzle through cutout opening...
Third, duct tape closed sink drains and basement heating duct and air return where applicable...Make sure all vents and windows are tightly closed...
Next, Start leaf blower to pressurize camper’s interior and using a spray bottle mixture of soapy water, spray all seams and look for bubbles...
Note: While at it, check tail lights and jack mounts and all appurtenances..
This method is usually successful but may be a bit problematic where roof insulation is comprised of ‘tightly fitting’ styrofoam blocks - in this scenario it ‘may’ be difficult for the pressure to communicate through the blocking to certain roof seams.