Couldn't states take an odometer reading at tag renewal time and charge based on distance traveled? It wouldn't perfectly capture usage of in-state roads, but it should be close enough on average, it reuses existing mechanisms, and it doesn't require any sort of location tracking.
Washington state has a pilot to do this, it will be mandatory in a few years.
Individuals would have to track miles out of state or off of public roads.
It sounds like it would be easy but states would open themselves up to endless nickel-and-diming or fraud.