Wow, and sorry but I don't have all the answers:
1. Don't know as I use a crescent wrench on the plug (This pack of replacements says 1/2"
2. Don't remember as that was a few years ago when I picked all that up--just went to Home depot and bought parts that fit
3. Same as #2 just picked up some from the Depot and used what worked
4. I usually buy 3 gallons and always left with 1 full so yes: 2 gallons
RV is all covered and all put away for the winter--I can look the next time I visit it.