Hi, I'm not sure what you're referring to by 'fresh water drain tube and spigot.' If you can put a shut off valve inside the insulated portion of the RV and drain the line that is exposed to cold would probably be the best strategy to stop the frozen water spigot. The other direction is to enclose, insulate, and heat the space the water is going through. Insulation alone won't really help unless heat is introduced to the area also. In many RV's the wet bay, with all tanks, pumps, and drains are in a space that has a hot air duct from the furnace to prevent freezing. It's enough to keep above 32º but not enough to heat the water in the fresh water tank.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI