Ah, OK. That pic helps tremendously. I see why you're saying the hatch door is not ideal. OK, I'm going to answer as if I was doing this to my boat, so this is just my opinion...
After I knew what mount I was going to use, I would cut two pieces of 3/4" plywood to be at least 2" larger than the mount. Then epoxy the pieces together (or possibly a 2x10/2x12, for example, depending on mount size). Hold this wood piece up to the area you want to mount and trace it onto the headliner material. Cut the headliner and remove it. You're going to epoxy the wood to the fiberglass (after sanding to get rid of the majority of the headliner glue) with thickened epoxy... but first make the wood look nice. Once it's mounted, you can now use 1" or even 1-1/4" screws to secure the TV mount to the wood. That will be VERY secure and there is no protruding hardware to the outside.
When you are done, this will look very nice and will not detract from the aesthetics of the interior in any way.
Possible ideas to make the wood look nice...
-- Just paint the wood.
-- Fill, sand and smooth the wood with bondo, then paint.
-- Cover the wood with matching headliner material - stapled to the backside with SS staples... or even better, Monel staples.
-- Cover the wood with something else... white vinyl that matches the other vinyl in the cabin... use one of the other colors of vinyl... even black vinyl will look nice since the TV mount will be black.
I would cut the headliner a fraction of an inch smaller than the wood so there's no "gap" where you see the bare fiberglass.
All that said, is mounting the TV there conducive to watching the TV? Or would you rather have it somewhere else?