Design ideas for a platform to lower camper onto...

Unless it’s an old rickety camper, don’t worry about it and leave it on the jacks.
If you’re going to be using it, off the truck, and want more stability, lower it all the way down and it’ll be pretty darn stable. If that’s not quite enough, something easy to set it on like a half dozen pallets or similar is what I’d be thinkin.