I used some stuff called Geoguard quite a few years ago and haven't had any leaks yet. I believe it's an industrial roof patch, It goes on over a matting and I did the seams all around the aluminum roof and vents. The down side is that it's pretty expensive and the shelf-life isn't too long.

Yea, forget the tar, unless you want tar streaks down the side of the trailer in the summer.