Caveat all of this to say that around 1% of all vehicles on the road are electric. Throw in hybrids and that number doesn't increase much. EV sales are sitting at 3% of all cars, Hybrids are getting more popular with around 5%. So we're not talking about a lot miles driven that used to be gallons bought. Just by the numbers we're looking at a 1 to 2 percent loss in total revenue from fuel taxes compared to a time there were no electric or hybrid cars on the road. So the year to year loss is probably within the margins of error.
But I do agree with you that we'll have to figure out how and where to source income to fund the roads as more and more people buy hybrid and electric cars.
I don't see a mileage tax as a viable solution, not unless you do it for every car on the road. Not with how many people register a vehicle in one place but live/work/do most of their driving in another. As it stood before I bought an EV, when I bought fuel the taxes were going to the state of Michigan (and some to the fed). But now, a mileage tax (if Florida could figure out a way to certify my mileage every time I renewed my registration) would see all of that money going to Florida. That's less than ideal. (This also applies to a lot more folks than military- college kids, snow birds, folks who live in one state but work/drive to another, and those folks who travel extensively for a living).
I can see an added tax on out-in-town EV chargers as an option. Though as Hybrid cars drive up fuel efficiency we're going to have to figure out how to get them to pay their share- as they're not plugging into public chargers and they aren't using much fuel.
Now with an EV and a Hybrid the wife and I are using 20% of the fuel we used to, and are therefore paying 20% of the road taxes that we used to for the same amount of driving. So on the micro scale my household is paying significantly less than it was 2 years ago when we both had traditional ICE vehicles.
You're overthinking it. Gas and Diesel vehicles pay by the mile already in a round-about way via the gas tax. Some pay more, some less, but in general it's a usage tax. Drive more, burn more fuel, pay more taxes. Yes, EVs are a small percentage now, but the change will happen gradually over time and I don't think we should let EV drivers continue to get free usage of roads forever. It's unsustainable.
It's literally as simple as reporting your mileage every year with registration and paying a fee based on the mileage. Maybe at some interval you have to do an odometer verification in lieu of emissions testing, but at vehicle sale you have to report it anyway under penalty of odometer fraud. Likely different rates based on axle, vehicle gross weight, etc, just like we do for registration.
The only unsolved piece is federal revenues. I don't see them standing up a whole unit to process VMT taxes. However, ensuring federal dollars continue to flow to states for roads is critical. Interstates are national assets everyone uses.