I have had to reseal different trailers of mine. I used some kind of RV roof sealer but it didn't seem to hold up very well. It would get hard and crack.
Since then I found something better. Its called
BondaFlex Sil 199 PG. You can use a regular caulking gun to apply it then spread it out with a putty knife. To prep the surface I used denatured alcohol on a rag. So far I am happy with the results.
I got some directly from the Vintage parts department. Search the Internet. I'll bet you can find a place to buy it or just call Vintage.