looking for advice on floor replacement

Thanks for the good words, DrewE. I think you are right - no need to re-engineer the flooring.

I believe the water was getting in in two places: the very front panel was dinged and the seam was not sealed. I'm talking about the vertical wall meeting the floor, just above where the frame extends beyond the body. I caulked that yesterday and will be sure to keep an eye on it. Secondly, the front bunk does not seal well. The previous owner had to replace the wood on the front bunk, that should have been a clue that there was a seal issue here. It's especially a problem because the front face of the trailer is sloped (for better aerodynamics), but that means that water wants to drip inside. I noticed this because it rained after I got it into my driveway and the hinge area was damp. I will deal with that this weekend.

right now I am fully planning on gutting the interior and ripping out as much flooring as I can. Poking around, that there are extruded aluminum frames inside the floor where the wall meet. That made me feel a lot better that there is some frame structure there I can attach to.

My thoughts right now are to use marine-grade plywood for sure, top and bottom. Probably stick with 1/4" thick. simply going with marine-grade will be enough of an upgrade beyond builder specifications that I should be good for another 10 years at least. I will be encapsulating the edges and any holes with epoxy to prevent water from wicking into the board.

As far as the bottom surface - I'm not going to waterproof it beyond applying a water-repellent stain. I want the wood to breathe, rather than trap moisture. But, at the same time, I don't want any splashed water soaking into the wood - I want it beading up and dripping off. I expect to have to re-apply once a year but that isn't a huge deal. At the end of the day, I will be driving in the rain with it maybe a couple times a year - a few continuous hours of moisture followed by days of dry won't cause issues. Standing water being trapped against a moisture barrier seems like a worse situation to me.

Everything else on this trailer seems to be in great working shape. The roof is solid, the walls look good, appliances seem to be in good condition, etc. some canvas needs repaired/replaced, but that should be expected in a 10 yr old trailer.

If you do some searches here, you'll find a few tales of other people dealing with similar problems, as you suspect.

Before you repair the floor, make certain you've figured out where the water was getting in. Typically that's someplace where something penetrates the roof (a vent, skylight, etc.), or is attached to a wall (clearance light, window, etc.), or a seam where the roof and walls meet. That also means that often there's damage to the wall and/or ceiling structures as well as the floor. Take a good thorough survey of it all before embarking on a repair project so you know what all you're in for.

Also be aware that often the floor covering is applied before the cabinetry etc., and extends underneath them. You may well have to remove a good bit of the interior pieces of your trailer to repair things. The good news is that often they are more or less just screwed into position, so this isn't an impossible task by any means.

I would resist the temptation to reengineer the floor much if any. Thicker plywood would mean a heavier floor, and cut down on your capacity for carrying water or cargo. (I don't know how it is on your trailer, but I would guess that you don't have an excess of weight carrying capacity to begin with.) The laminated plywood and foam sandwich is quite stiff and strong when properly made and bonded together; it acts a little bit like a truss, with the plywood carrying the loads mainly in tension and compression, held in position by the foam. I'm not sure that aluminum joists through it would actually strengthen it much, and if they prevented bonding with the styrofoam they could potentially weaken it somewhat. (They'd also reduce the insulation value of the floor, but that's presumably not a big concern for a hybrid trailer.)

For making the plywood and foam sandwich, I'd probably use epoxy, possibly with a thickening agent for better gap filling, and lots of distributed pressure, applied from the middle and then working outwards. As I'm certainly no expert here, that may not be the best approach or adhesive to use. I definitely would suggest at least looking into using marine plywood; it's far more resistant to coming apart when wet than exterior or especially interior plywood, and usually is made from better quality veneers than ordinary plywood. I would keep the bottom waterproofed, though if there's a convenient way to do so it would not be unreasonable to leave a little airspace between the bottom plywood and the waterproof covering. The bottom will get quite wet anytime you drive through the rain, and is exposed to rocks and sand and debris that wear away wood finishes.

Pay attention to how the walls and floor attach. I think often the wall structure is built on top of the floor, and so any part of the floor that needs replacing underneath the wall will require some careful thought and work. There are many variations in such construction details, though.

This sort of a project is a rather long and difficult and involved one, but it's certainly not beyond the reach of a fairly handy and dedicated do-it-yourselfer. It may end up being somewhat costly; take a sober look at things and decide if it makes sense to repair the trailer, or to get out before throwing good money after bad.