michelle;
Was the person working on the LPG system able to tell you what brand of fuel system the truck has? 85% of LPG fuel system not starting are electrical in nature. Does this system have an electric fuel lock off which is controlled by the engine's oil pressure sender. Are all of the plugs firing, and most importantly what does the engine's compression appear to be? Where did you aquire the fuel from? Is the proper grade or is it a commercial grade of fuel. We need to go at this in a systematic and orderly manner otherwise we will end up chasing an imaginary tall tale.