When I got into RVing in 2006. The most noted 4 season TT was the Arctic Fox. I bought one.
Several things made that TT four season. First was the holding tanks were enclosed in fiberglass insulation and heated by the furnace. Second was the dump valves were cable operated from inside the area of the heated enclosed holding tanks. I did not have the thermal windows that was an option at the time. It was very good a low temperatures.
The newer four season “options” I have seen do not address the valves in a meaning full manner. My current class C can be fitted with heating pads on the black & gray tanks. But the valves are exposed to the weather. Putting a barrier under all of it does not make it fit for four season use. After seeing what went into making my Fox.
