Well, we have 80% of a floor.
First thing today was to drill the pockets on the cross pieces and get them screwed down. When I was going to do that, I noticed the 2x6's weren't snugged down to the frame so I added some carriage bolts vertically through the frame rails.

Cut the first sheet by lining up the front left corner both across the front and down the left side, and marking from underneath on the right side, then flipping it, sawing, and flipping again. Then I lined it up on the back and both sides, letting it overhang the front, marked it underneath and flip, saw, flip, and check then screwed it down with a screw on each corner. Once everthing's cut and fit together, I'll take 'em loose one at a time and TB2 'em back down with 2 1/2" screws along all sides and center supports.
First sheet down. 43x79.

Before I monkeyed with the second sheet, I laid a piece of 2x4 across atop the tire, and found that with no wheel wells I'm going to have just under 3/4" of air above the tire. No bueno.

Laid another flat 2x4 on the framing, and got just over 2" of space. Poco mas bueno. Once I applied Newton's Posterior Law to it (sat on it), it bowed the spring less than 1/4 inch.

If my 210 lb only does that, I think I'm gonna be okay with 2" clearance. (Check the clearance, Clarence?) [Proofreading my post before hitting "submit", I just realized that there's gonna be an additional 5/8" of clearance: The main floor plywood will be under the 2x4 spacers. So I'm probably really okay on Clarence

] Pic is with me sitting on it:

The second sheet went full width and covered the wheel wells just about centered. Did the line-up, mark, flip, saw, flip, and screwed it with 4 screws:

After stabilizing it with the 4 screws I went under the trailer and marked the areas I'm going to cut out over the tires. I'll flip it and cut out the wheel holes tomorrow.
I did climb up on it, from one of the front corners, and it is solid. Very little frame warp when I stepped up on the unsupported front corner. Probably zero after gluing the deck on it.
I did get to do a little surfing, though. Was walking around on it, getting pics, and my dumb ass got a little too far back and achieved a negative tongue weight

just after taking this pic. It was an exciting few seconds as I got it to slam back down on the tongue block.

The partial sheet I had was 6 inches too narrow to fill in the tail. I It's a half sheet, but because of the way the cross members lined up, I had that strip cut off the front piece of plywood. So.... back to Lowe's tomorrow. The trailer was laid out originally to have the front sheet come all the way back to the front of the wheel wells, I expect, because it was 48" outside-to-outside on the crossmembers at the front edge and in front of the wheels. By adding 6 inches to the front, it threw most of it off. I don't think it was a bad thing, because I wasn't sure if I liked that joint that close to the front of the wheels, and having the unsupported little strips on the outside of the tires. It's always stronger to cut a hole out of a piece of plywood than it is to notch it and have a joint for one side of the hole.
Plan is to rip some 2x2's to fur up the wheel wells and then lay a piece of plywood on top of that. I'm not going to do that permanently until I drag it around loaded like I expect it to be, plus some, and see if I see any scuff marks on the bottom of those covers. So that's on down the road.
Tomorrow: Cut out tire holes, rest of floor, rip some 1x1's to give the sides a "stop" to push against. Those might or might not stay after the walls are on, depends on how glued to the walls they are. I seriously doubt that a 3/4" high "baseboard" is going to be any sort of a hindrance

To put the nasty on the bottom, I believe I'm going to brush it on. I'm going to spray paint the outside of the can of pooky flat black and leave it in the sun all day. It should be pretty thin after that, don't ya imagine? Then I'll use Scoop to pick the tongue up high enough to get under it and start at the back and work my way forward with a brush that I don't mind throwing away later. Might even offer the son a bottle of water if he'll paint it on

(Running joke.... I ask him to do something hot and unpleasant and offer him ice water as pay, and his response always is, "well, Dad, I wouldn't need the bottle of water if I wasn't out there in the heat doing you a favor!"

....maybe I'll offer him a

)