Camper Odometer.
I wanted to keep track of miles traveled. I could do it with a log, but then would have to remember every time. There are "hubometers," but they're made for semi-trailers and the tire sizes they use, and quite expensive.
So, I got a cheap bike computer (Sigma 400) off eBay for <$20 and mounted the sensor to the brake backing plate, and a rare earth magnet on the wheel hub. That lets the sensor be a bit farther away, and sense better at the higher speeds. I used Shoe-Goo to hold stuff on, which is pretty stiff and strong when it hardens. The magnet clears the lip of the backing plate.
The wire just follows the wires for the electric brakes, then along the frame. The computer fits inside a plastic soap holder I glued on, which gives it more protection from the elements.
I previously had a Sigma BC506, but changed to the 400 when I did my axle upgrade.
The one "gotcha" with the BC506 is that when setup according to directions, it will keep total distance in kilometers, even when set to show MPH. The way around that is to set it up as if the tire size is 1/1.61 (the ratio of km to miles) its actual size. That works out to be ~1326 mm circumference. Then it will totalize miles. If set to display speed in kph, it will actually show mph. (it can give trip average speed, distance, etc., but that's not something I expect to use much)
The 400 doesn't have that issue. The new tires are ST225/75R15, which are a little bit larger, and the circumference for those works out to 2259mm to track miles properly - there's well under 1% difference between it and my car.
The battery is supposed to last 2-3 years, and this unit allows you to set the totalized distance, so you can check it then reset it when changing the battery.
It seems to work fine so far, but for my first trip (with the BC506) with it I didn't know about it recording km, so the distance was high by 1.6x. The 400 works as it should.