Towdometer

I've seen adverts for used cars saying things like "100,000 miles, 30,000 towed behind RV". This seems like a good idea since the wear on the car would be unevenly distributed by the lack of engine use for so much of it's life. Car manufacturers could offer a towdometer as an optional extra. This would be an auxiliary odometer that only functions when the engine is inactive. This way the claims of the RV users could be verified upon sale of the car. It probably wouldn't be an especially popular addon but since it would require almost no special parts; just an odometer of existing type and the switch to disengage it, it shouldn't be prohibitively expensive to install. It needn't even require a space on the dash since it would only really be of interest when selling the car. A small, unused spot under the hood would suffice.