Given your occupation I'll assume you aren't scared off by some work and have all the skills necessary to complete the project.
For the vent, it the leak coming through the sealant or a cover. If it's a 12"x12" vent it's an easy and cheap replacement.
If you're confident there's no rot I'd reseal everything using RV specific sealants (NO Silicone). If you're not sure if there's rot, push on all the walls to see if there's play and walk the trailer (especially the corners) looking for soft spots. If there's significant rot this will likely be a good indication, but isn't a fool proof way to determine water damage. If you can pull the ceiling tiles out and see that the roof is in good shape you're good. Do you smell any mildew or see any signs of water damage inside, outside or underneath?