I had a new Speedometer (with odometer) installed in a truck I had years ago. It was before all the electronic stuff of today, but even then, I had to give the mileage that was on the previous odometer (or at least a close representation) to the DMV. They are, for the most part, aware of what the mileage is.
Then, there is also CarFAx (though I don't know if anyone had it for an RV?), which will document the odometer readings for each event.
Yes, I guess there could be some fibbing, but if you document it, (say with a label on the dash "Add 30K to current reading", you would probably not have to much trouble....especially in about 5-6 years.
As long as you don't appear shady...you probably won't have troubles....though, as some have stated, you may be reducing your buyer pool down the road!
