Best Sealant to re-seal RV side hatch covers etc.

In my opinion, any kind of caulk is for touching up around the edges. If removing the item, clean off all the old stuff and apply butyl tape. You know it's sealed when the stuff squeezes out when re-installing. For a neat look, scrape away the excess and run a bead of Dicor; but it's not necessary to make it water tight.