I foresee an excellent sales job, resulting in Father-in-law footing the costs of the whole project. (And a certain individual whose initials are DC doing most of the work.)
Sales points; storm shelter, root cellar, security "backup" position, storage, food pantry, emergency exits, blackout haven, work area, ...
Other thoughts -- the ATV access could also be a way of moving stuff in or out during construction. If you want to not fill up the airspace with noxious fumes, remove the fuel and the spark plugs from the ATV, and "run" it on the starter, (short spins down the tunnel and back should not overstress it.)
Or possibly arrange another twelve volt motor to be the power plant for the tractor. You could rig a little cart with a twelve volt motor and reverse current to change direction. Ana would love it.
You might want to keep this project secret if you can. It would add to its security function to be so.
Natural gas available in that area?
I'm following your project plan with interest and feel compelled to note that you may not be the first person to have thought of burying something important underneath that road.
Don't forget to locate underground utilities before you procede. We don't want to see you go and actualize your screen name.
Find the green bar of corn in the center of this photo. Those 5-6 houses on the left side of the road are directly across the street from my in-laws property. If you knew what you were looking at, you could find my Camaro sitting out back by the field. The big white mass just south of that green horizontal bar is the in-laws house and the concrete flower garden my father in law built.