Personally, I think I would only consider a Northern Light or Bigfoot. Ideally with double pane windows. I would mount the TC on a flatbed as previously stated. This will allow you more usable space to carry stuff; extra propane, fuel for generator (if not propane), etc. I would consider plumbing the propane into your existing system. I would use aluminum tool boxes that opened from the side, and were removeable. The flatbed would really come into play when summer camping with the family. It would provide more storage.
The flatbed makes it easier to add up to 4" (or more) sheet insulation on the lower sides and wings of the TC. I would sandwitch a 4" sheet of insulation (with 4" tall wood frame) between the TC and Flatbed deck to help insulate the tanks. I think keeping the tanks from freezing will be the biggest single challenge.
I would use two Wave3 heaters, one mounted and the other portable. In a TC, I think the lowest setting on the Wave6 is too high for a TC unless super cold. You will need to install a low amp fan to blow into the basement.
Although not as cold as what you will experience, I did live in my TC for four nights a week in the parking lot at work. I did this for five years. Minimizing your electrical (Amp Hours) is absolutely necessary. You will get a lot of theory, but I highly suggest a Trimetric or equal and accurately measure your usage. Anything to reduce your AH use is your friend in the weather you are talking about. You do not need to replace what you do not use, solar or generator.
Maybe I am expanding to far away from the question of which 4 season TC, but those things I suggest could make a borderline 4 season TC a great 4 season. I would not even consider a TC with a slide. For your primary use I would go with an 8 to 9.5 foot Northern Light or Bigfoot. I might consider a 10.5 foot but it would be less efficient in my opinion.
I am bias, I have a 11.5 foot Bigfoot TC. It took me years to find one I could afford (used). I have had other TCs, but not with slides. My last preference with siding would be filon. I would go to great lengths to avoid it.
Wayne
