I have had three products recommended by friends that have used them and really liked the performance of each. I have yet to use any of them and don't know which I would choose if the need should arise.
http://www.tapecoat.com/other_pages/roofseal.html
http://www.eternabond.com/?Click=205...FQw7gwodnie_2g
http://www.campingworld.com/shopping...-sealant/21208
Good luck, whatever you decide!
Remember
good cleaning and preparation of the surfaces in question is paramount to a good job.
JIM