I was told that the switches (labeled "Engine Diagnostic Switch" and "ABS Diagnostic Switch" on my coach) have nothing to do with the data port, at least in the case of the Silverleaf data stream. They are used for some other testing. I get all the data that I am supposed to get with the switches in their normal off position. I assume the Scan Gauge D would be the same.
The power pins on my diagnostic port are always hot, even with the chassis power switch turned off. As the Silverleaf JIB has LED lights that would eventually run down the battery, I used a source that is only hot when the ignition is on to power the JIB. Not sure if the Scan Gauge D would have the same situation.
'04 Alpine Coach 36' MDDS