I have a small spiral notebook in the console of the truck and note all expenses, miles traveled when refueling etc. Then it gets put in a spreadsheet that I have been keeping for the past 5 years or so....average MPG, Total Fuel Cost, AVG Cost of fuel, Miles traveled and States covered and notes for anything interesting - special campgrounds etc....I also rate every entry from a quality standpoint on a scale of 1-10....
I refer to it often when trip planning and when going to the same place or close for places missed or places really liked.
It helps....
Jim