Thanks for the suggestions! I think mounting the spare on the tongue would be easier, so if that is the way to go then I don't have to worry about extra blocking. But nevertheless, I did mount the 2x4s under the galley if I decide to go that way.
As for storage box depth, yikes! I did want it fairly deep and only considered that it would be higher than your typical speed bump. Before the bottom is glued in place I will give a lot more consideration to the depth. Thanks!!
So now a continuation.
Since I have my 2x4 framing lying flat, I have a broader surface for butting plywood edges. SO I decided to attach the floor in a few smaller sections, Of course the whole underside will be treated later.
Here is the front panel. It is glued and screwed in place.
And here is the back one. These floor panels hang ever so slightly over the edges and will be trimmed exact with a router.

And here is the fill in.

Still needing to cut the storage compartment cover, drill for bolts, flip, seal for roadworthiness, flip, and bolt down.