I would submit a Thor Customer Care request using the VIN of your coach and ask if they can send you the engineering drawings of the battery compartment or at least explain how it is constructed. If you explain that you are trying to move from 12V to dial 6 V they may send you the drawings.
If could be a crap shot, however, given the age of your coach as to whether or not Thor has or will send the drawings.
You can submit the request here
https://thormotorcoach.com/customer-care/