Landing gear is connected directly to the battery. Bypasses the disconnect switch as a "last resort" ability to raise/lower the jacks.
The truck charge line is on the house side of the disconnect.
With the disconnect switch in the off position, the truck was feeding the lights and other interior 12 volt items. The landing gear was pulling off of the battery and just ran the battery down.
Recreating the situation will just help destroy your battery's ability to bounce back from a deep discharge. DON'T.