Might be time to check voltage at the battery with a multimeter to make sure. Sounds like the batteries may well be shot. Can you have them load tested somewhere?
The red light on my switch illuminates regardless if the switch is in van or house mode when the radio is on. If in van mode and the vehicle is off, the light is not illuminated and the radio is off. Sounds like the switch might be bad.
When the radio is on house batteries, it can drain the batteries. Good chance you have an Amp that is powered from the house batteries when the radio is on. Shouldn't drain that much overnight though.
Nice looking van.
2008 GMC 3500 Quigley Weldtec 4x4 Savana SMB