Welcome to the Forum!
I have an Arctic Fox travel trailer. I would consider it a 3 and 3/4-season rig. It has R-9 in the walls and R-19 in the ceiling. The furnace, when running, is ducted around the water and holding tanks to keep them from freezing.
It doesn't have dual pane windows, and I wish it did. Many Canadian models do. You can put the "plastic wrap" type of film on the inside windows to help insulate in winter.
I wintered once in it at 7500 feet with no power source other than the solar and a couple of generator hours daily with a Honda EU2001. Yes, there was condensation, and when it got below 20 degrees there was ice on the inside of the windows, but I made it. <g> I also skirted it with one-inch foamboard and that made a BIG difference.
Here's the trade-off. A well-built trailer is much heavier than the "vacation" class ones. Unless you are just going to sit in one place and have a "pro" move your trailer, you have to consider a tow vehicle. At a loaded weight of just under 10k, my Fox is at the safe towing limit of my 3/4 ton truck.
Also, the higher end brands do not usually make anything shorter than 34-36 feet. If you only go to RV parks or the wide-open spaces, this will not be a problem. However, if you like primitive campgrounds or boondocking in the National Forests, the larger length can be a serious hindrance.
Also, I looked at high end SERIOUSLY 4-season fifth wheels by Teton. They are made extremely well, but only come in fifth-wheels and are not towable by a 3/4 ton truck. Some even require a medium duty truck to pull them.
New Horizons is another excellent brand. They are know for fifth wheels but I have seen one travel trailer by them.
Excel is another excellent brand, but again, usually fifth wheels only.
If you could tell us more about how you plan to use the RV, we could give you better information.