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.