On our 04 Imperial the Aladdin system keeps track of the stats. I just went out and checked, the coach, since its born on date has traveled 101945 miles at an average of 50.6 mph and average fuel mileage of 7.7. It also keeps individual stats for a trip setup on the computer. It saves the last 10 trips and provides the same info as I mentioned above for each leg. It's an ISL 400, weighs in at around 33k lbs without tow which adds about 4500 lbs. I reset the Aladdin on a new trip when I fill the tank and use the fuel consumption feature to tell me when I need to stop for fuel. The fuel gauge is useless and has failed several times and I'm so used to not using it that I'll not fix it again.
As an OBTW, I've logged a few trips over the central plains where a tail wind had allowed me to travel at 70mph ans still see an average of 9.5+ avg mpg. Wifey and the fuel fund both pray for tailwinds!!
Ron