Sounds like the regulator may be losing all the propane that is normally kept in the line between the lockoff and the regulator, or......you need to be pushing the manual primer button on the backside of the LPG regulator for cold starts.
If this truck has a Model J Impco regulator/vaporizer, there is a flat aluminum primary valve inside. This blade shaped valve has a little square of rubber in its center which seals to control liquid fuel entering the regulator. Typically, deterioration of this rubber will permit liquid fuel to enter the regulator even when the primary diaphragm has it pressed shut.
In that scenario, after the engine is shut off, liquid fuel continues to leak past the rubber face valve and given enough time it will vent all of the liquid fuel that was in the line coming from the lockoff valve.
Something you can do is to loosen the fitting on the liquid line at the regulator BEFORE you attempt to crank the engine for the the first start of the day.
If you do not see any liquid fuel escape then the blade valve inside the regulator is leaking.
If you DO see some liquid fuel escape, you probably need to press in hard on the primer button located on the backside of the regulator to give a shot of fuel into and past the secondary chamber so the engine has fuel available as soon as it starts cranking.