It is interesting how varied our different experiences are. I have designed a very simple checklist/log. It fills a letter sized page. On one side I have the checklist for "take off" from each campsite. On the other side there is a corresponding number for that campsite where I enter the location, site number, any comments about that site and campground. I keep the original, but enter a summary into an Excel spreadsheet where I keep up with the total mileage on the RV. I have used this same system for four different fifth-wheels which we have owned and used over almost 20 years.
I do not keep the mileage records as we travel, but work up the mileage based on the actual route we have taken as I enter into the spreadsheet. Not a conscious decision, just the routine I've fallen into.
I have also begun to keep copies of an annual calendar on which I have brief notations. I wish I had begun this sooner. I produce the calendars with Microsoft Publisher.
BTW, in writing this post I checked my Excel logs. On our present HitchHiker we have 59,421 miles. I won't go into the others, but I did come up with a total of 146,359 miles.