I'm not sure about drive through car washes for a dually, I kinda figured that if it was low enough to fit, then the extra wide fenders didn't matter, but in my case I just go to the self-serve wash once every 2 weeks and wash my truck, use about $15-$20 to do it, my truck probably can make it through the automatic one, but I'm too worried that my horns or compressor will get screwed up by the undercarriage wash.
In terms of the mpg this winter, I'dve seen what some of the tech members have written about it being lower due to the winter blend fuel.
-BG06.