Chuck,
I was mistaken. The 50/50 mix of water and boiler antifreeze provides heat for the coach. That is providing your coach heat. However w/o water in the system if you have the water heater switch on you may burn out the electric heating element in the water heater. It would be safer to refill the water heater. If you have the unit running and have the T'stats set for ~50 or so you won't have any freeze issues.
Again, sorry for my confusion.
Steve