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.
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.
where do you even find 1" ply? I can't imagin how expensive that would be....
