On my 04, and I think your 05 is the same,
The batteries are located at the Right Rear of the coach. (Passenger Side)
Open the door and you will see the pull-out tray with 6 batteries on it.
Two (connected together) are for the chassis that powers the starter.
The four connected together power the house to provide lighting etc. in the house.
On the dash you will also find a button labeled "BOOST".
If you hold the boost button down while trying to crank the engine, it will connect the Chassis Batteries to the House Batteries and unless they are all dead, should give you the cranking power to get the engine started.
Good luck, if you need more information, don't hesitate to ask.