Inside the RV? Anything you would use in your own house or bathroom. Typically any brand of silicon.
Exterior? I've tried a good number and find I prefer Dicor sefl-leveling as well as their non-leveling formula for vents and windows on the sidewalls.
More unique for my camper are the fender skirts. Nothing would adhere to both the fiberglass wall and the plastic skirt and water would sneak in past the skirt and into the camper. Until I tried 3M windshield urethane. WOW. That stuff is amazing.