Here is a couple of Pics to try to explain what I thought about..
My fuel flap is on the road side, and the storage door to the water tank and underbed is right in front of it.
It occured to me that if you could drill a small hole, say 1/4" through the rear frame of the storage door frame, into the fuel flap, you could put flat headed pin or nail in it to engage a tab on the fuel door.
That way, when the storage drawer was shut, the pin would be held in, and it would be locked.
A bit of accurate drilling required tho, and the is no tab on that side of the fuel flap, so I would have to TIG one on...
Plan B, would be to use the tab on the other side, and have a spring loaded pin, with a release cable from inside the storage door or even the water flap...
Sorry if this seems a bit crazy... Vicodin is not my favorite way to fly!