We had it happen last year on our Winnebago (Freightliner). Like you we write down the mileage when stopping and I also do it when fueling up. Pulled into a station for fuel and when we started back up again the mileage was wrong. It didn't go to zero but to but to 1341. Nothing you can do about it. It is illegal to change it back and you can get the real miles from your onboard computer. The only problem we have is with our extended warranty people. Even though all of this has been reported to them with authentication by a repair shop they still question the mileage every time we have work done.