In the good old days, you could disconnect your battery and the vehicle would still run off of the alternator. With sophisticated electronic circuits, that is no longer the case. In fact I remember a specific warning on one of my vehicles about disconnecting the batter while the engine was running, and the damage to the electronic circuits that it could cause.
AT first I thought that this seemed to be an ignition switch problem, similar to just turning the key off. The thing is that on most rigs, that would have no affect on the headlights, IF THAT is the lights he is talking about losing. IF he is talking about losing all of his dash lights, than it still could be an ignition switch failure.
Hard to say or guess without knowing the wiring diagram of the type of chassis involved. The chassis battery shorting out to ground would have the same affect as turning off the ignition and may also prevent the alternator from providing any power and may also cause the headlights to go out. This is as simple as a complete inspection of the chassis battery, or main wiring circuit to see if there is a frayed wire from friction or rodents, that could short out the main 12 volt circuit to ground.
