No power from Battery to Camper

[buckshot]

New Member who just purchased a 2002 Montana Mountaineer, not a fifth wheeler, I have a fully charged new 12 volt battery and it is correctly wired, there is 12+ volts at terminals but nothing works in camper unless I plug into ac land power then things work, I can also hear (when plugged into land power) what sounds like a fan running just under where the fuses are in the hallway so I assume this is probably generator or inverter. Any ideas what might be going on here ?

Am electrically illiterate so answers that understand that would be helpful.

Thanks,
Richard