Fantastic information Catmandu! You certainly know your stuff.
Manitou, When people talk about a 4 season motorhome, i think there are a few other models that have good insulation and good construction handle winter's low temps like the Chinooks and Born Free, just not using water or septic. Which most people don't use in winter anyway. I don't think there are many dump stations open up in the northern area in the winter and that would keep me from using the toilet, shower and sink. Your right, the Bigfoots have heated and sealed holding tanks and each compartment even the dump valve compartment is individually forced air ducted. There is no hanging or exposed plumbing or tanks when you look under a Bigfoot. The other features that gives these the 4 season classification is the foam injected Fibercore wall, roof and floor system, R12 walls roof and floor with R8 underbelly. There is even rigid foam insulation in the aluminum frame tubes. It works well, keeps the heat in when cold and keeps it cool when hot out. They came standard with double pane windows. This solid construction also helped when looking for a class C that could better take some minor offroad and flexing.
Another comparison that might be worth checking is the size of the holding tanks and furnace size between the BF and Chinook, for some people being able to stay out for an extra couple days could be the difference.
Ours has the 460 with 4:10 gears and its fantastic! Loaded running on the I-15 the best i got was 11.5 mpg, and out of overdrive she'll maintain and even increase speed on large hills. I recently bought a complete, very low mile 7.3 Powerstoke diesel motor with the thoughts of just maybe doing a swap but i'm really re-thinking the idea because of how well the 460 runs.
The 20-21' whether, nook or BF are so nimble, i do not feel like i'm in a class c when driving it, so far i go and park in the same areas i did with my Sportmobile.
I really like the Chinooks and the sleeping arrangements was really the only major factor in passing on it, they other features of the BF are great and make a fantastic built RV but were not the end all be all.
Fall of last year there was one of these BF 20' for sale in Revelstoke BC, it had very low miles but the exterior looked liked it had sat outside most of its life. She wanted a lot of $ for it at the time, i know that it didn't sell and was put back into storage. But this is the only other one I have ever seen listed.