At this point I have no idea what my fuel mileage is and I would like to begin to get a handle on that for planning purposes. The bus starts easily, does not smoke at all, and runs great. Having mileage in line with norms would just further indicate that the engine and drive train is in fact running properly. Knowing somewhat what my mileage is would help in planning costs for future trips.
Knowing the quantity of fuel in the tank is not necessary to check fuel mileage. You simply need to be consistent when filling the tank. Always fill to the same level, whether you use the dip stick, or the bottom of the fill tube. Then you know exactly how much fuel you burned in a given number of miles and can easily calculate the rate of burn. With that, and knowing what your total usable capacity is (120 gallons), you then know what your maximum range is and can plan accordingly.
If you really want to know per inch, then level your coach when the tank is nearly empty, empty the tank of all fuel using a siphon or pump, and fill the tank in increments of 10 gallons, dipping and marking your dipstick with each increment. This will be very time consuming, but will be the only reliable way to get an accurate dipstick, and it will be accurate only when the coach is level (not very often).
An alternative is to calibrate the dipstick for mileage. Fill to your known full point (e.g. the bottom of the fill neck). Run a specific number of miles (e.g. 50 miles). Dip the tank and mark the dipstick. Do this for 800 miles, dipping every 50 miles. Now you have a mileage calibrated dipstick, rather than a gallon calibrated dipstick. And isn't that really what you want to know? How many miles can I go on what's left in the tank?
Incidentally, if you fill the bottom of the fill neck, you will often find that as the fuel expands in the tank you may see dripping out of the fill cap. This is especially true if you park on the curb where the fill side is slightly down hill.