Fresh water is always needed in a camper. With as much time as we spend in the desert, having a big supply of water isn't really optional. Technically speaking, a 12-gallon tank is barely adequate for 2 days, if we stick to the 2-gallon per person per day rule. (Don't worry, we supplement with bottled water and a 5L "Jerry Can" for longer trips.)
Given the construction of our Craigslist-sourced conversion-van bench/bed, we had just over 8" of clearance underneath for a tank. Fortunately, Valterra makes this line of excellent ABS tanks. Each tank is 8" by 16" by "X", where X is some incremental length from 9" to 72". It happens that 8 x 16 x 24" fits very nicely in the available space, and nets out 12 gallons.
The real advantage of the Valterra tanks, from the DIY perspective, is that they're ABS - meaning it's very simple to put fittings, fill-spouts, etc. anywhere you like. Compared to Polyethylene tanks that require specialized equipment to make fittings, this is a cheap and easy way to build a "custom" tank to fit your needs.
Here, I'm marking out for a fill port.
Other parts of the tanks come with pre-molded bungs, but they're sealed from the factory. Temporarily adding a short nipple into the bung lets you drill out the plug without risking buggering up the threads.
Here you can see the vent-line (center), along with a "sight-gauge" on one end that lets me visualize the water level, even though the tank is opaque. The fill-port and gauge are oriented so that they're easily accessible from behind the seat. I'll be able to fill by running a hose in through the rear doors.
Here's a quick little test. I was really hoping to keep things super simple. This was to check the performance of just having a valve, no pump. The results were, predictably, underwhelming. There's just not a lot of head pressure here.
Rocket Hand Pump arrived, I moved quickly to plywood. The hand pump is plumbed into a "tee" from the same line that feeds the faucet. This way, I can (slowly) drain the tank on gravity-only, or pump out water as needed. No electric pump again keeps things simple, though I'll admit filling the wash basin for dishes or refilling large drinks bottles gets tiresome!
The fire extinguisher was eventually relocated to the driver's seat area, but you can see the butterfly latch that holds down the "hatch". I made the rear portion of the mini-cabinet removable. It allows me to easily access the plumbing bits for service, but there's also enough space behind that area between the seat and the wheel well for me to store the coiled fresh-water fill hose and water-treatment supplies. Pop the hatch, pull out the hose, and I'm ready to fill the tank. The yellow cap is just to keep the valve clean and spider-free!