On all '70's Airstreams that I have seen, including several I have owned, there are heated tanks. That is there is a little 2" duct that goes to the tank area from the main furnace ducts. Often two, one for the water tank, one for the holding tanks.
Look for a 2" duct(s) that goes through the floor.
That said, the heating system is primitive at best. It will probably keep any problems from happening down to 15 to 20 F, but not much lower. That of course assumes all your heat is coming from the furnace, not some electric space heaters. The furnace must run a fair amount to push any real amount of heat through those little ducts into a big uninsulated (or poorly insulated) space where the tanks are.
The holding tank outlet pipes and valves are almost never heated or insulated and enclosed in Airstreams, so the "heated tank" may not freeze, but the outlet valves commonly freeze. Not pretty if they freeze solid and break something.