You didn't say what State you live in. I live in Pa. and have a "Shoot from the Truck Permit" (SFTP). In order to use it, the vehicle (a vehicle can be a car, truck, ATV, lawn tractor, or any motorized vehicle) must be completely off any public roadway, at a complete stop, and the engine shut off. Only one loaded gun is allowed in the vehicle and only the permit holder can shoot the gun.
In order to obtain the permit, an application is obtained from the PGC web site and completed by the individual, and another part completed by a Physician. Then the form is submitted to the PGC for their review, and if THEY think the permit should be issued, it will.
Dave
PS--I have not covered the use of ATV's on public land. Just because you have a SFTP doesn't mean you can drive your vehicle or an ATV anywhere you want to go.