IF the weight of a couple of extra passengers is enough to make your drive uncomfortable you may be uncomfortable anyway. By filling your water tank you MAY loose up to a quarter of a MPG and DOUBT if you will notice any performance differences. You would GAIN the ability to stop whenever you like for a meal or bathroom break and avoid the need to find a place to fill up on the road.
All that being said, you can usually fill up at ANY campground, public or private but at many of them you will need to pay a fee of some sort (usually aboutr five to ten times the amount of extra fuel you would burn by carrying water). State parks will allow you to use the dump station and fresh water fill by paying the "DAY USE" fee for the facility. Private RV Parks usually have a set rate for dump station and water fill.
Good luck and enjoy the trip / Skip
