That sucks! Someone else posted this past week a similar issue with their grey tank valve.
If you can do a little snooping and figure out where the valve is located, you can cut the underbelly to gain access. Recommend cutting a hole at least 1 foot by 1 foot (if your confident of the valve placement. Or go bigger like 2 foot by 2 foot. Only cut three sides of the box, leave one side (side closest to the front of the rig) uncut, use that to make a flap. Once repaired or "fixed", tape the flap shut.