I have both AC and DC power on the skoolie. For the DC side, I have 4 deep-cycle marine batteries yielding 900amp/hr of capacity. I built a heavy duty slide-out and store the batteries in one of the belly bins. I charge them using 120VAC (via extension cord) or with the alternator (65amps). I designed the system to be able to live 'off-grid' for ten days using about 50% of the battery capacity.
I used an RV integrated converter/battery charger/AC/DC system. I also have a secondary marine DC-power circuit breaker (Blue Seas) with Xantrex LinkPro power monitor. I avoided using any of the skoolie's original wiring or distribution. It is, simply, a mess (see photo above at lower left).
I have a 70-gallon freshwater tank that is stored under the bed and a 45-gallon grey water tank located in a belly bin on the starboard side. I use a DC-powered SHURFLO pump rated for 5.5 gpm, The water inlet is located on the back of the bus, starboard side.
The shower was built in the space that was left between the kitchen cabinets and clothes closet. Half of the shower is located over the wheel well so I just turned that into a seat. There isn't much head room in the shower so a seat really helps. I used a vinyl shower pan, covered that with 1/2 cement board, and then coated that with Red Guard, (the water proofing material, not the Russians) so I'm confident the shower will not leak.