> The other empty space I would like to utilize is between the frame 
rails on the sliding door side.� Either a tank there, or tool box or a 
trap door storage area?>>>>

I made a long wooden box out of 3/4" plywood, with a swing-down hinged door and lock at one end. Bolted up to the floor in the space mentioned above.� Painted it flat black.� I store two metal one-gallon gas cans end-to-end in there; one is gasoline, one is Coleman fuel for my camp stove.� Lined it with thin carpeting and a foam pad on each end and between cans for a little padding.�  Used to carry both inside the vehicle for years, but decided that in a worst-case scenario it would be better to have those fluids outside the vehicle.� A little hassle to access (have to kneel down and reach under) but lots of peace of mind to have a "reserve tank".� I empty the gasoline into a nearly full gas tank once a year and replace with fresh.� No issues so far.� Plenty of room to make a "triple" or maybe two "doubles" I think.

Rich
San Diego