after looking through the information i have found on this there has been like 6 different revisions done on this fuel system.
What information i could find is mainly parts info, not much on service information. Since it is a dual fuel system the 2 solenoids you see are for LP lockoff and most likley gas lock valve. The one you see on the mixer is probably an idle solenoid? Since the mixer/carb is playing a dual role here. Also should be a switch located somewhere in the same area for switching it from LP to gasoline, the solenoids are controlled by this switch, its function is to switch circuits to energise either the LP solenoid(s) or the gas lock valve for gasoline operation.
Lets not forget about the LP safety switch valve, a flat pancake looking switch with a vacum line. If this switch is not working properly the truck will not run for long or at all.
I guess you have tried in both modes LP and gasoline and get the same results?