In all my years of camping I have pulled off the Xway a couple of times. Its very dangerous plus if there is any traffic at all, it is very difficult to get back on without getting run over. I always fill the water tank. In very cold weather I just fill it with hot water and take off. After all, that is one of the nice things on having a RV. I want to use the bathroom any time and I like hot water to shave and wash my hands. Anybody who lugs a pail of water around for flushing when they are in a rig that cost big bucks are just making things hard for themselves. Her fiver probably has heated water tanks.
My advise, fill up the tank, hit all the rest areas, and have a nice trip.