We bought a pretty standard RV watertank, 28 gallon capacity, from an online supplier. We basically maximized size for available space under the bed. All the plumbing is with brass fittings and PEX using the clamp rings that require a tool to install. This yields a flexible but durable system that will survive a lot of bouncing and is actually somewhat freeze resistant too. Advice from other travellers pointed us strongly towards PEX (or Hep2O in Europe) for the piping/plumbing. Brass fittings are just more durable and easier to re-use / work with if things need to come apart. We used a standard Shurflo water pump that we put on a power switch so that it could be turned off when driving or for any reason.
All of the pressurized side of the plumbing – the water pump, two water filters, the faucets, and an additional valve for connecting a shower hose, are contained in a very small area and are all above a floor pan that drains through the floor of the van to the ground. The sink drain hose goes down the same hole in the drain pan. Essentially, any plumbing leak on the pressurized side will be contained under the sink and will drain down into a pan and onto the ground. Coming from the tank to the pump there are just one valve and two PEX fittings, minimizing the risk of leaks.
As far as what plumbing we actually have, coming off the water pump we run to a valve for a shower attachment and into a household water filter that takes standard 10″ cartridges for a rough clean of the water. After that a line goes to the sink and a line goes to a Nature Pure QC2 purifier for drinking water. Each time we fill water we put a tiny bit of unscented regular old bleach in the tank to keep it clean, then the rough filter and the purifier clean the water up for us, meaning that as long as the water we get is reasonably clear and is free of chemical contaminants, we can turn it into safe drinking water. This is a huge upgrade for us over our prior system of 5 gallon water jugs and having to seek out good water to fill with!
Our shower solution is a simple hose out the side doors of the van with a shower head, and a tarp to turn the open barn doors into a shower stall. We thought for a while about hot water but decided it wasn’t really worth it. Camping in the lowlands we don’t need hot showers, in the highlands hot showers are generally easier to come by, and we didn’t want to give up the space to the water heater.