For the diagram try the search section on this forum. I know its there somewhere because I used it to check mine last spring. As for gas mileage it depends on your weight, terrain, how you drive, weather, traffic, and if you have a good running engine or not. I have an 87 Allegro 33' with a 454 and weight of about 13,5K and get about 7mpg. I am looking to redo the carb and a couple of other items and maybe get to 8.
2017 Grand Design Reflection 303 RLS
1997 GMC Sierra 3500 Dually