I would go with the three frames of 2 x 4s and probably 1 inch ply on top. You already need a little more height and there is a big difference in strength between 1/2 inch and 1 inch ply.
1” plywood?? Difference in strength??
what you’re suggesting does not make sense. For multiple reasons.
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.
Since my old truck had 2 1/8" clearance at the and never hit the cab in 20 years it would seem that 2 1/2" would be more than enough. Both the old truck and camper are 2000 model years and I'm pretty sure the Bigfoot was designed to go on that truck. Is the concern that the new truck will flex more than my old truck?
With regards to the additional strength of 1" plywood I'm not sure what benefit it would have. The way the camper is designed it has a subfloor that bears all the weight of people, appliances, etc. It has "joists" around the perimeter and forming the basement walls that transmit the loads from the subfloor to the truck bed. The water tanks are suspended between the joists and have removable access panels. So as long as I have support where the joists are the plywood wouldn't have any significant loading.
I'm just trying to keep the center of gravity as low as possible and minimize the extra weight of the platform mainly for ease of loading/unloading the camper. But if there is an actual benefit of a platform with increased height and weight then I'm willing to do it.
Here is a picture from another forum that shows the underside with the access panels removed.
