I always did the "ultralight" thing back when I was backpacking. While a van-camper doesn't have to worry about weight, the ultralight gear is also usually pretty compact to save space. So that may help out.
Since almost all my dwelling is urban and I can get anything I need in the Walmart any time I need it, I have only the most basic necessities in the van--a cot for sleeping, my alcohol stove and kit for cooking, a hand-cranked washing machine for laundry, a Lady Jane bucket for pooping, a sink made from a stainless mixing bowl (soon to be replaced by a plastic "fish-cleaning sink"), a small cooler (with extra styrofoam) for keeping leftovers, a couple of sweater boxes to store odds and ends, my day-pack--and that's about it. Anything else I buy as I need it.
I try to follow the philosophy of the backpacker--"bring what you need, and need what you bring".