on some of the earlier models(when peugot engines were first introduced in these trucks) there was a choke cable and butterfly assy that is inline with the airhose if i remember correctly, this was specific to the LP conversion trucks only if i'm not mistaken.
Later on they got away from this as the LP systems developed into better setups with improved components.

But it sounds to me like your issue might be in either the LP vacum switch or the LP lockoff solenoid. There is a slight possibility it could be in the converter/vaporizer but i'd check the other two things first.

good luck