I have kept a log/journal since my first truck camper in 1993. I briefly record each trip, where we stay each night, campground cost, any good/bad points about the campground etc. At the end of each trip I record the total mileage for the trip, not individual daily miles or fuel consumption. I also note service items like new tires, gas check etc.
I refer to it quite a lot, for such things as planning how many days to allow for travelling when I'm making reservations, but most of all for keeping a check on how many miles the 5th wheel has done, surprising how this mounts up.
Its interesting looking back over the years and comparing campground prices with todays prices!