I haven’t had to do either method.
But either will work just fine IMO. I think the biggest thing is to keep whatever method you use to build up from being able to shift or move around.
To that end, I’ve thought about it if I had to do this and I’d seriously consider attaching the plywood foam sandwich or frame to the bottom of the camper itself.
In years of owning older campers I’ve only had to get into the bottom of one, once. Water tank supports finally cut loose from being used off the truck with full tanks and only sitting on the jacks.
To my thought above, if ya did it right, the buildup attached to the bottom of the camper would not only be more convenient for loading/unloading but could also easily be designed to provide additional support to the belly and still be removable.
And unless you already have or want a rubber bed mat in the truck bed, you could attach the mat to the bottom of the camper as well and the whole mess just stays with the camper.