If the , salesman switch on my coach is flipped or the relay defective , all interior lights would operate with the engine running. The BIRD charging system would activate and by trying to charge the coach batteries , power up the interior.
Salesman switch is a different one than the aux start. Mine is inside a cabinet above the stairs to the entry door.
EDIT: Salesman switch, could ( should ) be labeled battery disconnect ; and only works on the coach battery. Shop worker may have hit the switch to keep your batteries from being drawn down, while waiting for you to pick the rig up.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.