Get yourself a Motor Carrier's Atlas to find which roads are approved for the 18-wheelers. I would stick to the Interstate as much as possible. In your case, after checking Google Maps, it appears you can do your entire trip on the Interstate.
Can't speak of the entire Yellowstone area but when we were there the end of Sept we learned that the town of West Yellowstone closes up shop the first of Oct. Nice thing is most shops will have their stock on sale before closing up for the winter.
You won't be winterizing as your MH will be in use. Be prepared for the park's water pipes to freeze during the night so you'll have to use your on-board water. As I recall we had low temps around 28*. And drain the water out of your hose to prevent it from freezing. Once the sun comes up things should law out.
How far we drive each day depends on if we are racing the clock to get somewhere. We went from Tucson to Tampa in four days doing over 500 miles each day. Some days we might only do two or three hundred miles. Depends on what we're doing.
