Large trucks and SUVs tend to be so expensive to buy and operate that you can pay for a lot of deliveries and truck rentals per year—maybe a boat slip at the local marina, too—while still saving money.

Farther out in the sticks, sure, those things may be so inconvenient or unavailable that it's not an option, but it is for the average Suburban Assault Vehicle owner.

Yes, some people really need big vehicles (and/or we need them to keep farming) and so you need exemptions. In the UK there is Red diesel for example that is for farm use / private use that is much cheaper than diesel for the road.

What is the justification for this subsidy?

Likely untaxed for road wear that isn't happening when they aren't driven on roads.

Right—it's not a subsidy, at least in the US, because much or all of the gas tax goes to road maintenance, so if you're not driving on public roads, you don't have to pay it.

Doesn't make sense to make e.g. farm tractors that rarely see a public road, pay for public road maintenance when fuel's purchased for them. Or trucks and other vehicles (some farms are really big and may include road-worthy vehicles that hardly drive on a public road more often that the tractors do).

If you're going to only drive on private land, you also don't need tags/plates for your car, at least around here. No registration. Just personal property tax and you're good to go.