Hi,

I still suspect something is wrong with the LPG system.

Anyway...

Is the selenoid on the truck black or blue?
Should be blue, there were serious problems with the black ones.

Try to remove the piston from the LPG valve under the selenoid. The valve opens/closes when activated by the selenoid.
Without the piston, you should have the permanent LPG supply.
Check if the engine works.

PS
Is the timing set properly?