Well, after having owned airplanes, keeping an RV log seemed obvious. It probably helped that the previous owner left me his log. I log all my maintenance, as well as trips.
I do it on the computer in Excel. I use Excel and the Good Sam trip planner to plan a trip. Then I write my mileage on my gas receipts. After the trip, I copy the worksheet to another worksheet in the same file, labeled "Actual", and update it with the actual numbers from my receipts, including the RV parks we use, and any attractions we visited. I print that sheet and add it to the 3-ring binder that holds my log.
Wow, it seems a little obsessive when I describe it. My wife thinks it's nutty. But what can I say? I'm a computer guy. For me, the statistics are part of the fun.