Try replacing the large solenoid on the mixer, if you have a 'known good' one. they stick sometimes, usually when new. if this is a new truck, or the computer has been changed there is a "learn" procedure that needs to be run with a laptop computer.
ALSO what is the temp when filling the LPG tank, and is the tank filled from a bulk tank (outside) without a pump? (gravity feed). if you fill the tank when the tank is warm, (say from working inside at 3 PM) but the bulk tank cold, (from sitting outside) then when the truck sits outside overnight, the pressure may be changing quite a bit.