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

If you are just planning on smearing some around the outside, use whatever is cheapest - it won't last very long regardless. If you are going to remove it, clean it up, and seal it under the flanges, then I'd use a good polyurethane or polyether marine caulk. Then you won't have to do it again next year, or for the next 10 at least.