There's a lot of overly strong absolute posts here.
A full water tank is a lot of weight and in the case of our camper does affect our ride and performance and mileage.
If we NEED a full tank of water- we don't hesitate to haul it.
BUT
The rest of the time (camping with hook ups) we do the 10 to 15 gallon thing.
Sort of the common sense elusive middle ground...
By the way, to the OP- you did see the arithmatic: 20 gallons of water = roughly 160 lbs- so it's like having one person on board- which I agree- does not make or break your performance.