Roadrunner,

Have you looked in the bay where your hotwater heaters, water pump are curbside bay cloest to the rear wheels. You might spy what looks like a sewer dump valve onloy the size of a grey water dump valve. If it is there, it should be labeled "fresh water dump". Is there a chance to leave you genset on and run either you webasto or electric heaters. There should be wabasto exchangers in your underbelly as well as one electric.
Good luck. YOu really don't anything to freeze. It will be a ROYAL Pain you know where to fix.