The residual water in my shaded garden hose is considerably cooler than that what replaces it from the mains.
Seconding PNW's notes. NOT a good idea having hundreds of pounds sloshing around at the highest point!
Don't know where it hailed from, the prior owner bought it before his fiancee torpedoed the Skoolie dream, but, I have a roto-molded fresh water tank that fits perfectly between the batt box and the ladder frame rails.
Kinda small tho, at 30 gals.
Those who say that it cannot be done should not interrupt the people doing it.