Originally we put all the "get set up" stuff back there. Then we started thinking about weight and g-forces when bounding on I-5 and I-10, among other obstacle courses we've travelled. Ballistic weight may matter.
We moved the 30 amp electric cord to the truck bed, only store a 10' sewer hose and it's related paraphernalia in the bumper. Pretty light weight there now.
The rear compartment went through a few iterations (everything does when changing things around is so easy.) The only things in the rear compartment are a pair of flamingos (mostly feathers and air, right?) three tennis racquets (definitely light,) a few spare rolls of paper towels, a couple of Aerobies, and two cardboard tubes containing spare parts for my HF antenna (bulky, no weight.)
The side compartment (also under the bed) is a little weightier with two bags of leveling blocks and a couple of bulky things.
We moved all the heavy stuff to the truck bed - it's got the weight rating, frame and suspension to carry it. The trailer just doesn't.