This is unfathomable to some, but there were many campers built that did not have battery boxes. Mine, a 2000 Palomino B1500 popup, being one of them.
These campers were designed to run off the vehicle battery through the 12V charge line of your 7-pin trailer plug.
On mine the converter is also the breaker box and fuse panel. The 12V from the truck comes into the converter and powers the 12V items when the converter isn't plugged in, and carries charge current back to the battery when it is plugged in. Find that wire, and that's where you connect your batteries in.
