I'd second the comment about double paned windows. If the coach doesn't have them, don't even consider it in the cold.
As far as Alaska goes, here in central Alaska, NO ONE stays in an RV during the winter unless they don't mind being cold the entire time, from October to May. Very dangerous to try to keep it heated with Big Buddy heaters too. Just not done very often. Down in Anchorage along side the coast they might try it more often.
Just as an example, the Jayco Insignia Class A has a R-14 insulated and heated underbelly, R-19 roof, R-10 walls and dual pane insulated windows. Something equipped like this can be considered an "Artic" of "Polar" package good for below freezing conditions.
Contrary to the popular belief that the Canadian built Triple E Class A RV (and Triple A RV's) are "extreme weather" units with extra insulation, Tripe E does not list the R-Value Insulation info...however to quote from Triple E's Class A Brochures, they use 1 inch and 1 1/2 inch styrofoam in the sides and floor, which would give it a R-Value of 7. The roof is R-10. They also list a "thermal break material is used" in the walls, which could be foil. But again no insulation value is listed! The Jayco would have 2 to 3 times more than the Triple E top of the line Empress model Class A! Some folks brag about the (gone but not forgotten) Big Foot RV's "winter capability" as well, which just had R-8 injected polystyrene insulation.
So look for the polystyrene or styrofoam insulation, dual pane windows, fully enclosed and heated underbelly. Some Super C rv's are extreme weather equipped as well, Gulf Stream Class C's for example! Good luck!
I have not looked at a Jayco Insignia but I have looked at several Jayco Seneca motorhomes. While the Senecas are enclosed and heated the dump valve area is not insulated. Under the dump valves is a moulded pan that is not insulated. It is only about 1/4" thick and is either plastic or fiberglass. but it is not insulated.
The descriptions about polystyrene or "styrofoam" insulation are not correct. Bigfoots used injected urethane insulation. It has a R-6.5 per inch rating.
There are two types of polystyrene insulation. Expanded polystyrene bead insulation (bead board)is the common type that you see in RVs. It is R-4 per inch. Many people incorrectly call this styrofoam.
Extruded polystyrene insulation is R-5 per inch. If it is the Styrofoam brand it is the extruded polystyrene produced by the Dow Chemical Company. Their marketing color for Styrofoam is blue. Owens Corning also produces extruded polystyrene under their Foamular brand. There are a couple of other manufacturers. One uses a green color and the other white.
Few manufacturers use Styrofoam, extruded polystyrene. Most use bead board because it is cheaper but also has a lower R-value per inch. Bigfoot is the only manufacture that I know about that used injected urethane foam.
I will either second or third NEWMAR for cold weather or hot weather use. Their insulation and wall thickness are great.We have been in weather down into the teens and not had a bit of a problem. Make sure and get dual pane windows. They make a big difference.
2003 Newmar DSDP 4005 dual slide king bed Spartan 350 Cummins 2004 Jeep Liberty 4X4 tow, SeaEagle boat in basement
My Newmar Dutch Star has the extra insulation package for the roof. I have spent two winters in Indiana with temperatures down to -8 degrees and I kept very comfortable. The motorhome survived the winters without any problems.
I can't tell you what to buy, but I CAN tell you that our H.R. Scepter is one of the worst for keeping the heat out or the cold in. I would give it a poor rating in this regard. Besides poor insulation, we have the huge curb side windows which contribute to the problem
'07 H.R. Scepter > P A Q
Cummins 400 ISL
Alison 6 speed World
Sparky the Wonder Dog
Without hesitation, I can recommend Triple E motorhomes for cold weather use. Made in Canada, where we know what cold is all about! Well insulated, insulated and enclosed tanks, double pane windows, etc.. Build quality is superb. These should be good value as well, as the US dollar is stronger than the $CDN.