Your answer lies in composite panel construction.
They are quite rigid and light for their thickness, certainly compared to 1/8" thick aluminum sheet. I have some panels that are 2" thick, weigh one pound per square foot and have an R value of about 30 with a special heat reflective coating sprayed onto one side of the panel.
This may not be required for a small truck camper, but is definitely superior to any single material such as plywood or aluminum sheet.
The skins can be made from a wood veener, sheet steel or aluminum, fiberglass, phenolic, ABS or any number of other materials. The cores are typically honeycomb based (Plas-core or Nida Core type products) or an EPS/XPS foam based. Each has its advantages and disadvantages.