Insulation & RV's, It's one thing to have floor, wall and ceiling insulation but one must remember that you have slideouts, most times three of them. The only thing between inside and outside is a rubber gasket. While looking at your prospective RV climb under the slideout and stick your finger between the slideout and gasket and you will get an idea of the opening that has just a rubber gasket closing it, on my kitchen slide I can see inside, I've never felt a draft but I wouldn't say it is insulated. My Bighorn was built in 2011 so I don't know if changes have been made. I think this is very common accross all brands, some do better then others with more then one gasket. No one ever seems to mention this issue.
It's just part of the design, you definately don't have house construction nor insulating.
Can you be comfortable, yes you can, many fulltimers live in the Landmark, Bighorn, and Big Country brands without problems but there is an art to it. You can call Heartland and speak to the product manager for your Gateway and get your questions answered, just give them your Vin# and they should be able to help you. It might take a couple of working days to get back to you.