the anti-diesel solenoid (as you call it) is actually an idle solenoid if memory serves me correctly. I would start with that. I've seen those solenoids cause many different symptoms such as this. Also there is a toggle switch that lets you switch from gas to LP, check those circuits and make sure its activating the idle solenoid and disabling the LP lockoff and visa versa.

Then go to the fuel pump and make sure its working properly. If it runs while choked and then fails when choke pulls off it sounds as though its not getting enough fuel in gasoline mode.

My curiosity though is why would the customer want to go with gasoline when lp runs cleaner and is cheaper than gasoline?