Conservation only gets you so far, and DW isn't fond of cg showers. For gray water (assuming you can get to a dump station):
I got two 5-gallon buckets with lids and pour spouts from the local tire shop (bead lubricant.) Buy a sewer cap with a garden hose nipple and a short piece of garden hose. If your sewer outlet is high enough you can fill the buckets, cap them off, and haul them to the dump station. You may have to lay the buckets on their side to fill them.
A step up: I got a 30-gallon plastic drum ($10) and it rests on a cut-down, upside-down pallet (free) in the truck bed. It's outfitted with a ball valve, vent, and garden-hose connector. I use a HF 12V marine pump to fill it with gray water. It gravity drains through a short piece of garden hose at the dump station.
Working on stage III: I scored a NIB macerator pump off Craigslist. We'll be 10 days w/o hookups and I plan on using it if the black tank gets full, pumping it into the 30-gallon drum.