2x4s and plywood is going to produce a VERY heavy camper. Campers are generally built with much smaller lumber. I can't imagine you're going to meet your weight goal with such an overbuilt rig.
If I were scratch building I think I'd look for a rotted and/or damaged camper to obtain cheap for parts.
You'll get windows, a sink, a power converter, stove, jacks, lights (inside and outside) etc.
Otherwise I don't think you've got enough money, and I'm a resourceful type that does things a whole lot cheaper than almost everyone else on here.
I've been in a lot of junkyards, you won't likely find truck camper stuff there.
2017 Northern Lite 10-2 EX CD SE
99 Ram 4x4 Dually Cummins
A whole lot more fuel, a whole lot more boost.
4.10 gears, Gear Vendors overdrive, exhaust brake
Built auto, triple disc, billet shafts.
Kelderman Air Ride, Helwig sway bar.