How do you track your mileage on your 5th wheel/trailer?

My Ford truck knows when a trailer is attached and tracks the mileage for me. When I traded in my travel trailer recently I simply reset the mileage counter though I could have easily added the new one as a separate trailer. It can keep track of multiple trailers, however, you have to remember to switch it manually to the correct one. It doesn't have any way to recognize which trailer I'm hooked to.