I believe that OBD was developed around the computers and sensors that monitor the engine mainly for emission purposes - none of which we have on our 12V trucks.
Because our mechanical engines do not have a specific engine computer, the info you will see from a generic OBDII scanner will be very limited. I use an AutoXray scanner (OBDI & II) on my '96 and can only access tach, speed, intake air temp, TPS (auto trans), and % load. It does display a coolant temp but it always says "-40". It also shows my engine is not running. I get more info from a 24V due to the ECM.
AutoXray is working on a Chrysler-enhanced upgrade (for manufacturer-specific codes) that may allow more parameters that are monitored by the PCM to be scanned (like voltage, battery temp, auto trans info), but you won't see the info you get from a gas engine because it is not monitored.
Bob