That thermo-lite looks very interesting....
Could one just use a 1/2 in piece straight away without a frame for a bed? Just some cross-members below.
I've been looking at Thermo-lite for the top layer on my panels. Some things to keep in mind if you're considering it:
1) Its fairly expensive. $4.50+ per sq ft in the 1/4" thickness. The pricing varies w/ panel dimensions and densities, but $4.50-5.00 is representative for 1/4. So if you're thinking of a top/bottom sandwich construction, you're talking $400-ish for the Thermolite. Pricing tracks somewhat linear with volume, ie 1/2" sheets are nearly twice the price of 1/4" sheets.
2) Weight wise, its on par with marine plywood, perhaps heavier. A 4x8 of the 1/4 inch 32 lb/ft3 density Thermolite is about 21 pounds. The 6mm okoume marine plywood I'm looking at is 18 pounds in 4x8. EDIT: the Thermolite starts getting a weight advantage as the thickness increases, since the middle urethane foam layer begins to account for a higher proportion of the volume; in other words, 3/4" Thermolite is lighter than 3/4 plywood, maybe 50-55 lbs versus 70 lbs. per 4x8.
3) I'm very skeptical the 1/2" panels are stiff/strong enough to bridge the ~ 68" van width without a frame. You'll need some sort of frame. I think that 1/4" top and bottom with a foam middle, sandwiched in a frame of 1.25" .125" wall welded aluminum C-channel, or in the appropriate 8020 extrusion, could work in 2' x 6' panels without cross members (ie no windowpane frame members). Or the 1/2" on top of a frame with no windowpane cross members if the frame is 1.25 or 1.5" aluminum. But until someone tries it, we're only guessing at rigidity.
I looked a SIP panels as a frameless alternative. SIP = Structural Insulated Panels sometimes used in buildings. I found a local guy w/ 18' x 6.5' x 2.5" thick SIP panels, but he wouldn't sell less than full sheets. I'm confident a product like that could work without a frame (though it would need some sort of edge treatment). They are fairly light, too, but the thickness wasn't attractive to me. Every inch counts for me (bikes underneath and 6'3" person above, hoping to sit up in bed) so 1.25 inches is the bed panel height to beat for me.
I'm leaning toward Thermo-lite rather than okuome plywood, but I don't feel strongly about it. I have reasonable access to Thermo-lite and I'm not on a budget, so $260 on Thermo-lite versus $160 on okuome isn't a big deal to me. I have no doubt that the Thermo-lite is stronger and I just sort of prefer composites.
EDIT/PS: I think 1/2" Thermolite with unwelded cross members of 1.25" or 1.5" square aluminum tube 0.125" wall could work, with cross members on 18 to 24" spacing, if the Thermolite is mechanically attached to the crossmembers. Rather than bolt all the way through the aluminum, one could drill holes every 12-18 inches and place rivnuts in the top wall of the tubing, and then bolt the Thermolite to the rivnuts. The edges of the Thermolite (near the van walls) would not be supported, but the strength/stiffness of the 1/2 Thermolite should be more than sufficient for that 18-24" gap, especially since those panel edges see the least force.