Need to raise camper 3"...looking for suggestions

I have used the pink insulating panels and they are cheap, lightweight, waterproof (unlike wood), and can be cut with a box cutter. Home Depot sells it in 1" and 2" thick 4x8 sheets. Wood is not a good choice where it is going to be exposed to the elements and swell and rot over time.

I put a mat over the foam and it works great. No need to overthink this unless you are bored.