Fixing LEAKS on 5th Wheeler

[tinkering]

Hi,
We were given a fixer upper 1990 ish (will enter the exact info a little later) Sprint Lite 5th wheeler. Some of the vinyl covered wall paneling is rotting inside so the first thing is obvious to me, Fix The Leaks
What is the best and most cost effective sealer to use on the roof, vents, walls, windows, and door frame? We are in Alberta so if somebody knows where to purchase the materials at a decent price in Calgary, that would be helpful too. I see some Eternabond products on amazon.ca but the prices seem high.
I'll post pics later too.