This style camper hatch may put you in trouble for rot:

Silversand, I have the same style cargo doors on my TT and they stay bone dry...or course I don't have the cut outs in the seal lip for the vent.

Maybe you need a new approach...instead of trying to keep the water out, what if you turned the propane area into a "wet" bay Line the bottom with a plastic or metal tray and route to a funnel or hose that drains out the bottom. Similar to DWeikert's set up. As long as the ater drains and wood areas are sealed so they can't absorb water you should be fine.