I always fill the fresh water tank, even if I will be camping in a place with hookups. If nothing else, it lets one use the toilet or wash lunch dishes while on the road.
Towing the jeep, assuming you're towing four down or using a dolly, adds little if any weight to the axles, which is what you mainly need to be concerned about. It also goes without saying that, say, fifteen gallons of water in jugs will weigh exactly as much as fifteen gallons of water in the tank. If weight is a big constraint, you could always fill the tank only partly.