Yes, I have a 22gal water tank under my trailer, over the axle to keep COG low, baffled to keep water from all rushing to one side when tilted, and armored to protect it from damage on the trail. I'm lucky it came with my trailer that way.
It has a 12v water pump pulling from the tank to an on-demand water heater, which is then plumbed to two points on the trailer; left rear corner with separate hot/cold for cooking and kitchen duty, and front right corner with mixed hot/cold for shower.
I love the whole setup. Having fresh water on-site via a faucet and shower hose makes extended base camping so much more of a pleasure. Traveling and camping solo, I can make 22gals stretch a pretty long time if conservative in my usage.
I'm going to add something similar to my van, though with the tank inside so I don't have to drain it/winterize it as often for freezing temps. That's something to consider if thinking of one of the options listed in previous posts that have the tank on the roof or bumper; you have to drain it and winterize the line if it's going to be freezing weather.
I also keep a
20L FRO Jerry Water Can with tap that I can move around camp or vehicles as needed. It's really a lot more handy than you might think.
There are a ton of places to get RV/camper style water tanks in a variety of shapes and sizes. Here's one:
Plastic-Mart RV Water Tanks. A search will reveal more resources.
I'd like to find a source for this kind of tank, from Germany:
but so far have not been able to find a good source in the US.
I would caution about adding too much weight to just one side or the other in a vehicle. Remember, each gallon of water weighs 8.36 pounds. Even dual tanks--one on each side--unless drained at the same rate or refilled before traveling again, could be a problem. Twice as much plumbing, too. If you have a full tank of any size on only one side, but balance it on the other side with regular cargo, then you need to keep the tank filled when off-road.
Another option I'm considering is a wide flat tank to go between my wheels wells, but inside, not outside. There's a 24 gal tank on the link above that is 44L x 26W x 6H that could work. Then box it in. I'm not overly concerned about potentially losing 6" of vertical interior space, so this may be the route I eventually go if I decide to add a tank to my tow vehicle.
Good luck with your project.