where do you even find 1" ply? I can't imagin how expensive that would be....
It’s available. 1-1/8” iirc. In the concrete formwork world. Maybe score a junk piece or 2 of old Finn form residential basement forms for cheap or free, but otherwise yes a sheet of it is expensive and unnecessary.
if the OP wants lightweight, sturdy and economical, foam is the best bet.
If not suggested here, it has been about every other query like this.
get the right combo of sheet(s) for height. Cap it with a thin sheet of plywood if want to spread the perimeter bearing enough to not point load the perimeter of the camper/foam and be done with it.
Quick, easy, relatively cheap. Or make a frame out of 4x4s. Strap the corners together with barn door type hardware and also easy, cheaper than foam and not overly heavy. Also can build to fit snug in truck bed so no chance of sliding.
