Marv,
For my money, Forlivier and RR have the real answer.
For everything I drive, I write the Odo and trip on the receipt. With the light things, it is just to have some idea. But for the coach, I find it essential to know that the engine condition is good (It had trouble a while back) and to know what my real tank range actually is.
We travel with a laptop, and so there is a spreadsheet with the recent fuel data. When I used to coach for business this was important for tax records. While traveling, we find it valuable to know who soon we will need a fuel stop. When in travel mode to get somewhere, we are not the 2-2-2 types, so locating a good fuel stop can make a difference in a days travel.
Most fuel gauges are nearly useless even the ones that I carefully calibrated, the tank shapes just make it that way. If you have an average of a number of fill-ups, this can become very repeatable and dependable. Once we made a mistake and had to buy fuel near the edge of a high price location. After consulting GasBuddy, I loaded exactly the three gallons we needed to be sure to make it to the less expensive fuel. When you buy fuel like we do, pennies mater.
Matt