TTs with less than 4 season capability can be made for 4 with some thought and work. You have to start with a preferably wood framed unit with enclosed underbelly, this was what we made into a -45 rig. From a 3 season . Wintered over in Ontario, Manitoba, Alberta. Never froze .
Dropped the belly, heattraced all water lines placed waterbed heaters on under the tanks. Power for them was run up to where we could either plug into 120 or inverter so that they could be powered parked or on the road as long as the truck was running. Screwed pvc 'brickmoulding ' to the frame so we could add 1 1/2 foam insulation then replaced the belly. if the axles hit the frame extension , so what. its pvc and coroplast. A little dent
Single pane windows were dealt with by cutting 1/4" plexiglass to fit, remove handles- trimming them with foam weatherstrip, held them in with turn mirror clips.
Refridgerator lower vents must be restricted when temps drop below freezing, foam pipe insulation works well, it is easily stuffed into the louvers, taken out when temps warm up. To keep the fridge working, the outdoor coils must be kept above freezing, below 100f or so. A daily chore dependent on weather,
It is critical to ventilate moisture, use a dehumidifier to reduce condensation in the walls and on windows.
Even the best touted mfgr 4 season units cant compare with this .
