Like you we are planning to buy a Fifth Wheel. We have Researched a lot.
As mentioned many times in this thread, and past. Floor plans are the most important. Then frames an Insulation as you stated for winter camping.
Be sure to have Double pained windows, and heated underbellys where you tanks are located.
Some manufacuters make there own Frames in house, like mentioned Northwood and
Arctic Fox, Peterson Industies and Excel,(Also making NuWa Hitchhikers using floor plans from NuWa), and DRV all make there own Frames, there are a few more companies that make there own frames. The last two are upper price range 5ers used by Full-Timers, with lots of floor plans to choose from and will build to your wants, but again pricy.
We are looking for a two to three year old Fifth Wheel because, by one of three we mentioned, above, there is a big Depreciation cost when you buy new, the minute you drive off the lot, and a lot of the warranty issues have been worked out.
Hope this helps you in your search.
Enjoy the research its a lot of fun planning like we are.
Scott