I don't think I understand what you typed there.
"true reading"? The regulator is telling you the actual pressure in the tank, when it gets colder the pressure actually does drop. As the tank runs out of liquid the pressure will start dropping. When the gas pressure in the tank drops below the low pressure reading, the low pressure reading will go down no matter how you adjust it - there just isn't the gas there.
You could theoretically dial your low pressure up to whatever the high pressure gauge reads, but you will blow your keg long before you get there (assuming the regulator allows it and the lines hold long enough).