Joel,
The "preheat" light that Larry was referring to is an indicator for the intake air heater located in the intake manifold on his 3126 engine (a feature that does not exist on the C 12), and not a glow plug indicator.
The best way that I have found to preheat a C 12 in a 2000 PT is to turn on the Hurricane, the Preheat switch, and set the thermostat in one zone of the coach (preferably bathroom for me) high enough to keep the Hurricane circulation pump running. You can turn on the key occasionally to check the engine coolant temperature on your Silverleaf to verify the engine coolant temperature increase.
Gerald