Food Truck Compressor

Hi there, I apologize if this is in the wrong thread, but here goes :P I recently bought a food truck and the compressor works fine. Gets the cooler/freezer cold when plugged into shore power. The previous owner said that they would also run on the deep cell batteries that are stored in the rear of the truck. My question is, why doesn't the compressor kick on when I turn on the Main Power switch on the front panel when not plugged in to shore power? A friend of mine said that it could be bc it's not level, but its not on the much of an angle at all.