We don't use a water tank at all. We prefer to use one-gallon jugs (only). I guess that comes from not having a tank in most of our vehicles we've used for camping (a station wagon, five cargo vans, a Class B camper van, a VW camper van, a towable 25' sailboat and a truck camper). I also camp occasionally with my brother in his Sunlite and Northstar truck campers and we don't use the water tanks in those either.
I had a tank in my Class B camper van years ago but I realized I'd rather remove the tank and pump to have more storage space. At the time, we were doing winter diving trips to Florida from our home in Pennsylvania. The van would be too cold to fill the water system before leaving and we'd have to winterize before returning if we were to use the water system. Also, we had a lot of diving gear so storage space was sorely needed. That low-down storage space was perfect for our scuba tanks, weight belts, regulators and fins as it could be accessed via the rear doors of the van.
I have a water tank in my Hawk truck camper but have never used it. When I bought the camper I told the previous owner I didn't really need it and he offered to take it and the pump out and cut the price. (He had installed that tank in the Hawk shell and had recently bought a Grandby shell and could use it in that one.) But I decided it would be better to keep it than try to replace it if I changed my mind or decided it would be better for resale value.
I never had a water tank (or sink) in my Econoline van and put over 200K travel miles on that one. I don't have and don't plan to install either a sink or water tank in my Transit.
For some reason we don't use much water so we usually only carry two one-gallon jugs of drinking water. We also carry a jug for draining off meltwater from our cooler and we use that for the porta-potti, small wash-down tasks, ensuring our campfire is out before leaving, etc. We have perhaps another gallon of drinking water in the bottles in our daypacks and the truck/van's driving compartment. We also started carrying a
GravityWorks water filter after having trouble finding potable water on a trip several years ago. I have on occasion taken along an extra gallon-jug or two when we were planning to stay in one place and access to water wasn't assured.
We don't have a shower so we typically camp at a public campground with showers when needed, i.e., when personal wash-ups (with pre-packaged towelettes) are no longer enough. On the few occasions that hasn't worked, we've found an isolated place to strip down and used our
Simple Shower or even just a sauce-pan of water dribbled slowly over each other (that's always fun!). We used a solar shower (the black-one-side-clear-the-other 2.5-gallon bag with shower nozzle) on our sailboat but never found it useful for the other vehicles.
We don't use much water for dishwashing as we use paper plates. We clean and sanitize the silverware and cookware in a bit of boiling water after wiping them down and washing with a minimal amount of water.
I believe this works for us as we move daily and typically travel in areas where we have fairly easy access to water.