I have a basic excel spreadsheet to track preferred campsites, fees, etc. I use one worksheet to identify all the campgrounds I like (or want to try), and then a second worksheet to list my trips for the year (dates, when reservations were made, cost of reservations, etc.). This stays on my laptop as it's only used beforehand.
For mileage, I keep a small notebook in my visor. On one page, I list the miles driven per tank, on the second page, I list the amount of fuel filled per tank.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)