After sitting overnight and acting the way it is, I suggest you check the condition of the large diameter vapor hose coming from the regulator and spanning over to the metal pipe near the valve cover. These hoses will sort of "unravel" or split due the the robust vibrations of the engine.
A hose with a split in it can completely empty of all propane vapors after sitting overnight and the split will made it slow to draw fuel out of the regulator at cranking RPM.
You might have to remove the hose for a thorough inspection.
Are you pressing the accelerator pedal when you do your cold start?
If not, you may want to try holding the accelerator down while cranking as that can make it easier to draw fuel out of the regulator.