not to argue, but many of the travel trailers and most of the 5er's in my part of the country have heated, enclosed tanks (heated with furnace)
However, most that i have seen start in the 27-30foot range.
As soon as you say "all season" be prepared to fork out the dough. there is more to it than just heated enclosed tanks. there is thermal, dual pane windows, additional insulation, etc etc. I had good luck with my Komfort 27ft TT. I used it for two winters, living in it while working in the oil patch all winter. I have a friend with a Citation Supreme and that has been very good for him in the winter as well.
You are on the right track when you say you want to stay away from heat pads. they are a disaster waiting to happen. i have seen about half a dozen rigs burnt down because of them in the las few years.
Another big thing to look for is the drain valves for the grey and black tnks. Many manufactures have heated tanks, but put the drain piping and valves out in the open where they can freeze. the best system is where the valve is right inside the tank compartment with a remote cable to open/ close them.
Sorry i couldnt help with your specific question about a 21 footer