Sounds like the injector on the throttlebody is bypassing. Easiest way to check is to pull off the intake horn on the top of the throttle body and turn the key on.
When you turn the truck key on with the engine off there should be no noise or propane smell.
If there is I would remove and inspect the large fuel injector. There is an oring to seal the injector end to the throttle body that always breaks causing propane to flow into the throttle body at all times when the key is on.
If the oring is good I have also seen the assistant injector (looks like a large six sided nut with two black wire coming out the back) get stuck open causing the same problem. Unplug and unscrew injector from the throttle body and check the pintle, sealing surface and return spring.

Both of these issues will cause a no start as the engine is flooded with to much fuel to start.
Hope this helps