I only started RV'ing a couple of years ago. I thought of keeping a log of my travels, but at my age, it would not be of any use to me or anyone else. I've kept enough logs in my life, Aircraft pilot log books, maintenance log books for said airplane, record of every airplane I worked on and basically what I did on it (finally gave up keeping the work logs after about 20 years of it) and I do keep basic maintenance logs on the vehicles.
For the vehicles I use a stack of 4x6 cards stapled together with a HD stapler. I make entries with odometer, date and what was done. I am specific about part numbers installed and where the parts came from. I do all of my own maintenance, no one else touches my vehicles. I do keep crazy stupid detailed fuel records. In 2011 I bought a car that was difficult to fuel consistently and I really just wanted to get a handle on the fuel mileage. That turned into an Excel Spreadsheet with tabs for each of my vehicles. I track the date, trip odo reading, regular odo reading, amount of fuel added (always top off), cost, the brand, and location of the station. The spreadsheet calculates the mileage for the fillup, a running mean average and a running median average, plus total cost, cost per mile, and I have a notes column to justify unusual results such as pulling a utility trailer that kills the mileage on my little Ranger, or to note the beginning or ending fillup of a trip.
I can tell you that my little Pontiac Vibe (the car that started the spreadsheet) has averaged 32.483 mpg in the 492 fillups since I bought it in October of 2011. I had the Ranger long before the spreadsheet but I added it also, and it has averaged 24.977 mpg in the 160 fillups since 2011 also. The Motorhome is a 2006 Dodge (Mercedes) Sprinter chassis '07 Winnebago View, which I bought in September of 2015 and hope to sell soon. It has averaged 17.061 mpg of diesel fuel in the 43 fillups since I bought it.
In February I bought a 2003 Ram 2500 truck with a 5.9HO Cummins and 6 spd manual transmission, it is low mileage (89K) and will be the tow vehicle for my travel trailer. The truck has only been fueled 7 times since I bought it (not counting the initial top off a block from the sellers home) and has averaged 20.499 mpg in the 3000 odd miles since then. I'm sure the trailer will make this much lower.
Charles