I'd just use some silicone caulk - you can buy it anyplace, including your RV dealer. Check the caulking around the antenna mount, clean it and smear some around where it looks like it's cracked. You should go up on the roof every spring and inspect all the seams, recaulking where necessary. I also clean the roof every spring with rubber roof cleaner - can get this at Wally World or your RV dealer.
Okay, I went out and looked at the caulk I got for the roof last year, and it's not silicone - it's butyl rubber, so I stand corrected. I still swear by the rubber roof cleaner from Wally World, though.