I had a 81 GMC Midas with dual fuel. Sorry there is no such thing as a manual for such a configuration. There was a switch to change from gas to propane. As long as the propane tanks were turned on, that was all that was needed to change modes. There was a little chugging for a second or two when changing modes with the engine running. It was best to change after the engine was started. Trying to change when the motor was off would cause a week battery to go flat. A problem I had was that I had to change O rings in the propane system. After that, the system seemed to work well.
Propane had less power, and did not get as good miles per gallon. But back then, propane was much cheaper than gasoline. As I remember, I got 8 miles per gallon on gasoline and 6 on propane. I changed the exhaust to dual pipes and was thrilled to get one more mile to gallon. Good luck with your rig.
