Being a relative "newbie" with the Cornerstone being my first RV, cannot comment on the Tiffin product. However, I do have some experience with the insulation on the Entegra.
My wife and I were in in Watertown, NY at the Association Island KOA in early October of last year. While we were there, the winds started blowing off Lake Ontario at 50 mph for nearly 2 days. The temperature was in the high 30's. We went to bed the first night and I turned on the fireplace but left off the Aquahot floor heat and the reverse cycle. Waking up the next morning, I got my morning coffee and decided to go out and dump my gray water tank, forgetting the weather conditions. YIKES.
I suddenly discovered how well Entegra insulates their coaches. It was 72 degrees and comfortable everywhere in the coach - quite the opposite of the outdoors!
Currently, we are in a campground in South Florida. It has been sunny and in the upper 80's. The DW likes to keep the coach at 70. You can hang meat in there. My neighbor, who is a newbie, ask we if my air conditioners ever shut off because his don't, set in the mid 70's. Ours spend about as much time off as they do on.
I am very pleased with the way Entegra insulates their coaches - and that is they way they insulate ALL of their coaches.