...and I realized that floors...
Dave:
Keep in mind, I'm a woody, not a foamy. But I believe in building light (and strong).
This is how I made my last floor.
It has an internal 3/4" framework:
The gussets are temporary. They hold it together until it's flipped over and skinned. With this floor, I skinned it with 1/4" subflooring plywood on both sides. You could use 1/8" ply on the bottom to shave weight. The voids are filled with foam board.
It's remarkably stiff and strong.
The skinny cross members are doubled 3/4" plywood with the grain up, not flat to aid in stiffness. The plywood is there to support the vertical upright pieces in the galley along with the batter, 7-gallon water jug, and 54-quart cooler. The lateral 2x6"s are there to support the walls and provide "meat" for the attachment points to the frame.
The 1 x 4" cross-member gives support to the headboard that will be above it.
I epoxy the bottom and give the top three coats of an oil-based poly.
Tony