Whenever we go to the races in Joliet I travel with water tanks full only a 20 minute drive and buy water only if we run out. Headed to the race in Sparta KY about 6 hr drive no water hookup at CG bu...
I doubt a few hundred pounds of water makes any measurable difference in fuel economy. I always fill it up. I also bring bottled water and usually a 6 gallon portable tank if we're dry camping which is our norm. One of the big reasons our next camper will be a toy hauler is the 100+ gallons of water capacity.