I don't like Sta-bil... its alcohol based and hydroscopic; attracts water and doesn't help the problems created by ethanol. I use mineral oil stabilizers instead.
I've heard as bad as 10mpg out of the 6.0; thats with a good amount of punch it though. I wouldn't be suprised to see 16-17mpg out of a 4x2 5.3, higher if driven with a light foot.
Absolutely. Everything from washer fluid to tranny rebuild, when it comes time. Only time my truck sees the dealer is when some specialized equipment is necessary; alignment, etc.