I go old school and use Excel to track our maintenance (RV and Jeep) and fuel fills (RV only). I always have my laptop up front with me while I am driving so it is pretty convenient to use.
Other things in the same workbook: GPS locations of campgrounds, DirecTV settings for campgrounds we have been to, propane fills, maintenance schedule, repairs needed/completed, weights from truck scales. I heart data :)
--kev