I am always amazed at the people that claim to get 9-10 mpg in a 20k lb motor home with a Ford V10. I have over 34k miles and have a life average of 6.492 and a current tank of 7.596. On rare occasion I get this higher number, I think I know why, I fill up AFTER I get out of the mountains and am on the way home. The mountainous portion of the trip is in the previous tank, 6.492.
I own a couple service truck with the same engine that weigh in about half my MH, they only bet about 9.5 on a good day!.
Keeping this record is fairly easy, even without excel, however with it you can easily calculate current, 4 tank, 10 tank and lifetime mpg as well as cost per mile, and keep track of all your service. get really carried away and you can even link to your RV log book where you keep track of all of your trips.
here are the headers I use in my spreadsheet, they are separated by (,) commas:
Date,Start,Finish, Gallons this tank,Fuel Cost,Current MPG,Miles, driven this tank,Aver MPG to date,Current $/Mile,Overall $/Mile ,Total Gallons,Total Miles Driven,Total Fuel Cost, Odometer Mileage @ Event,Trip Notes,Mileage % gain/loss, Four tank average,Ten Tank Average,Cost/Gallon
Formulas are not all that hard. just leave only the Finish Mileage column, Gallons this tank and Fuel cost as well as the event and trip notes unprotected.