Kurious G, happy to help. Forgot to say welcome to the board and good luck with the new-to-you rig. As beemer mentioned, start on the roof...go over every seam and around every device like antennas, a/c, vents, etc. Many have used Eternabond Tape on seams and Dicor self leveling sealant on the roof. Most RV roofs are EDPM rubber, check to be sure yours is too...if not, check to be sure you use a compatible sealant.
Feel free to post and ask questions, if it's busted or rotted, someone here on the forum has fixed it.
Let me know if you have an indoor area to work on the trailer....if so, we might be able to work a deal where I bring up my tools and help you in exchange for letting me borrow the barn to fix mine....