Purchased a new fifth wheel 2 months ago been in the shop for numerous things, they are telling me to leave propane tanks open for them to work correctly , the tanks don't empty out completely
There will always be a little left in the tank. The cutover valve senses the pressure drop JUST BEFORE the tank goes empty and switches to the full tank.
If the first tank were allowed to go completely dry, such things as pilot lights would go out and/or air would be picked up in the system which might cause misfire on the HW or heater.