Not sure how you could connect an ordinary battery to that DreamStation....but I see they offer a battery accessory as option. Could you see if insurance will cover that being as you don't have power when boondocking? .....it is an exorbitant $300. !!
Edited in retrospect - I see from a subsequent post that a DC cable for the DreamStation CPAP is readily available.....which should take care of the OP's needs if he has ready access to a DC connection to his RV battery bank.....otherwise should be easy to wire in a connection ....
Ed