We just use the square base 20/25L water barrels that you can pick up from camping shops. Actually, I think most of ours are from buying distilled water in bulk. These get loaded in as needed dependant on activity.
My 2c is that I'd fit long range fuel tanks onto the vehicle before a water tank(still got a tub). When we go away wit the camper trailer, using the 65l water tank, we can get 195l onto the camper trailer and usually have 100l of diesel in the tub.
My 2c on water containers like you are considering is to get the biggest you can fit, but make sure they are firmly attached and also protected from the sun(algae build up). Do they have internal baffles to prevent sloshing/load shifting?