I sort of covered this on my build, but another forum member asked me to post up some pics of my water system so I thought I would just do a write up. I went with a simple cold water sink with a 7 gal fresh tank, under the sink inside the van so it doesn't freeze if you have some heat on. I have no black water tank, just a hose run underneath the van, so I can collect it in a bucket or tank, but I never do. We usually only wash hands or rinse out cups there.
First, a simple Home Depot bar sink, I don't have a link but less than $50 IIRC
Underneath, I put a 7 gallon Aquatainer from Walmart. Any water tank will work fine
The only other thing needed besides plumbing parts from Lowe's/Home Depot is a 12v RV water pump. For some reason you can pay a little or a lot for one of these. I chose to pay a little,
$50 from Panther RV
http://panther-rvproducts.com/FloJet-Tr ... 6-144A.htm
From any hardware store, you'll need a light switch or 12V toggle switch for water pump on/off. I actually used a residential light switch (the flat type) in black so that it matched my black receptacles and all fit neatly in a quad box. Double receptacle on the left, water pump (light switch) on right. You'll also need a plug for the hot water connection on bottom of faucet, a length of clear hose (6' or so), a fitting to mate your clear hose to the cold connection of faucet, and a couple of hose clamps. I used a pex fitting because I already had one. That's it for the fresh side. The pump comes with quick disconnect fittings. Take one to the hardware store and get the size of clear hose that fits it, sorry I don't know the size but this isn't rocket science. For the drain, you'll need a drain to water hose (PVC) fitting and about 4-6' of water hose.
Cut your 6' of clear hose in half. Run it from inside the water tank to the inlet fitting of water pump, then the other side from outlet fitting to the Cold side of faucet. Make sure your hose goes all the way to the bottom of the tank and won't work it's way out. I've never touched mine. Work out your drain however you like. I just drilled a hole in the van floor and ran it straight out sealed with sealant. No trap is necessary on this type of drain, there are no gases to trap unless you have a black water tank.
Wire your water pump to 12V (constant) with about a 5 amp fuse I think (the manual will tell you) and run the ground through the switch. That's about it.
I would suggest making the pump a little more accessible than I did. I can get to mine but I was trying to fit a lot of stuff in a small cabinet. I can reach the disconnects on one side though, so 'winterizing' this system takes about a minute. Simply turn the pump on, turn the water on, let it pump all of the fresh water out of the tank and when it starts sputtering turn it off and disconnect one of the quick release fittings from the pump. A little water will drain so if you can put a towel under it. You're done. It won't freeze and there is no pressure in the system. If you have hard winters I would blow the lines out with compressed air. Oh, also, I do not have a water inlet on the outside of my van. I just walk a water hose in to the passenger door and fill my tank before I leave. The least this setup has lasted the 4 of us was about 4 days, but it has lasted as much as a week or more. An inlet (which puts another hole in your van) would be really simple to add but I haven't really needed it.