Pro Flex Rv Flexible Sealant by Geocel. It is for RV's and remains flexible. You can buy it online or at a dealership. Ace Hardware used to carry it in white, clear and black.
Another choice would be OSI Quad. You can get it at Lowe's and Home Depot and it comes in different colors. It does pretty well on trailers and I believe it is a polyurethane based caulk.
Do not use silicone. The sun destroys it, it is very hard to remove and nothing else will stick to it not even more silicone. It seems to work down into the pores of the material that it is applied to leaving the surface with a slick feel. This is at least my experience with it.