Truckers pay for hundreds of thousands of dollars in fuel every year. They pay way more gas tax than you do.
Given a fixed per-gallon gas tax, if you assume a 3000lb, 2 axle vehicle that gets 30mpg is paying enough road tax to cover its usage, then an 80,000lb, 5 axle vehicle would need to consume 320 gallons per mile to be paying gas tax proportionate to its road usage.
They don't (thankfully), so it's not. Trucks are massively subsidized by everyone else.