I had a similar issue with our 2006 Newmar Ventana on a Freightliner chassis. During a trip, I noticed the odometer "lost" 52,139.8 miles. I called Freightliner and was told how the access the mileage stored in the engine control module. That was how I was able to determine the exact error. They said that there was no way to correct the odometer display and if the display were replaced, the new one would start at zero. Seemed strange but that is what I was told. I made a label to put under the odometer display show the number of miles needed to be added for the correct mileage and there was no problem when I traded it in. I have no idea why it happened and the number it reset to had no significance that I could determine. It was something like 1100 miles. I record the mileage at every refueling, so it happened between pit stops.