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

Do not use silicone caulking from the local hardware or home center store. Dicor is one of the best and most available caulks although you will probably not find it in local stores. For vertical application you need the non-sag formulation.

If you do a decent job of surface prep including removal of the old dried up caulk, the dicor should last for decades of use and often for the life of the RV.