With so many RVs being called 4-season, or even 3-season, there is no written guarantees that I have seen. Mine being a 4-season, I can see many things that are of good design, such as the routing of the heat duct piping. Mine has one right next to the water pump, several running in the enclosed,insulated belly, and a duct pipe that runs right through the kitchen cabinets. There is a smaller heat duct pipe that empties into the tank area. I do have a few flaws, that were easily corrected.
The actual season use, depends on where you camp, and climate, which can be too extreme to still be able to use all the RV systems.
Jerry