If the roof doesn't leak and it's not rusty, a good oil based white paint will provide the reflectivity you need to reduce the interior temps. If you feel your trailer needs a sealant coating, elastomeric products available from many sources work well and are long lasting.

I used the Kool seal product on a steel trailer I had. It failed in a dismally short time, and had to be completely sanded off before I could recoat with a marine grade paint. It was a mistake that I won't again repeat.