Outside the box approach would be to rent a concrete saw and saw out width and length you need for your dual wheels. Dig out 14" of dirt put some mesh and steel in the now level depression in your drive and get a few bags of sakcreet and repour the area you cut out. May be more work than you want to do but in the future if things change, you could always re-pour it. I am assuming you driveway is uphill and you are backing in and that you chassis clearance would allow this to work. You would need a plate of some type to put over the cutout when the coach is not there.