The tank in my truck is a "horizontal leg tank", so its flat on the bottom, and wider than it is tall. Picked it up from a Tractor Supply. It sits in the front of the bed, just behind the back window.
On the side, it has a 1" bulkhead which I attached a ball valve and 12v water pump. I use it to carry extra drinking water for boondocking or rustic camping. Keeping it covered with a tarp, it takes a long time to reach "air temp", filling it with ice cold well water at the start of a trip. The water pump has a 7-way trailer plug and receptacle, so it can either plug into the truck bed (5th wheel connector) or into the trailer cord (power comes from trailer battery bank)
Last year I got lax in treating the water and the algae grew in there pretty bad. This year, I painted the entire tank black with truck bed liner, and I've been treating it with chlorine every time I refill it, and extra dose when it's nearly empty. So far so good.
