Are these all new units? If so, go on each of the websites and get the construction information of the vehicle and/or go to RV shows that have these brands and talk to factory reps. Ask about the construction including R-Values of floors, walls, roof, spacing of floor joists, roof joists, etc. What kind of materials are used? OSB or Plywood? How thick? What kind of foam is used in seating (density). Take factory tours, if possible.
Go to dealer lots and look at used ones even if you're buying new. Looking at lots of used ones will start to give you some idea of how they hold up over time. They all look nice when new on the showroom floor...how they hold up over time is the key and while the care and maintenance habits of the owner can make a big difference, poorly built units with cheap materials are pretty easy to spot when looking through used units.
In my own research on this (not including the really high-value brands that were out of my reach) I always came to the conclusion that the top three were Tiffin, Winnebago, and Newmar. While I would give Tiffin the edge because of the incredible service stories I've read, any of those three would be tops in my book. I've personally owned two Winnie's and one Newmar. All good though I had far far better initial quality with the Winnie than the Newmar (the two new ones I had).
"Second star to the right and straight on 'til morning." Captain Kirk in Star Trek VI 'The Undiscovered Country'
2007 39K Winnebago Journey "The Enterprise"
Jeep Cherokee Sport 4x4