I love the easy solutions!
I've just accepted the fact that the 6.6 is a cold blooded beast. It is parked until April, but I like to start it every once in a while just to keep things moving.
I know the t-stat is good because the old one I pulled out moved just fine (I put it in boiling water on the stove). I replaced it anyway just to be sure. I did that last year when we were actually driving the bus in the winter because we were converting it.
My final conclusion is that at temperatures below freeznig, the 6.6 turns into an air cooled engine. Radiator or not there is a lot of iron in that block without a hole lot of displacement. To back up this theory, the bus had a Webasto heater in it when I bought. It never worked and I took it out. I know the bus had been used in the mines so the webasto may well have just been used as a sort of remote block heater, but with under 100,000 miles, but over 5000 hours, I think this thing spent a lot of time doing the 24 hour idle thing. That leaves the webasto as just being there to keep the interior warm.