Grace bituthene (roof sealant) will do the job for you. You can get small quantities at the building supply stores. It tuff stuff to work with (think Tar Baby and the Brier Patch), but once you get on there, it's its sealed forever. I use it for all sorts of projects. My son-in-law is plant manager for Grace and I get some of the rejects... I have to be careful when asking for some. last time I got 6 100' rolls to do a small garage roof.
On my last trailer, I cut some strips, stuck them on above the tires and used some staples to hold it in place.