You should be able to trust the auto switch over valve on the regulator. Do your due diligence and look at your tanks daily. Leave both tanks open at all times, indicator pionted at only one.
The best guage of partial tanks level in cold weather is to pour a glass of hot water down the side, feel with your fingers where the temp. diff. is, that is your liquid propane level. In hot weather, use iced water! Do not trust the frost line, it may be yesterdays
Propanes vapor, boiling point is -44.3 f. As you demand it to boil off, the liquid chills, lowering the temp and volume it can vaporize. When it can no longer vaporize fast enough to meet your demand, the regulator will signal 'Empty' even when it may not be, due to the nature of the fuel.
Its physics 101, no other scheme will do unless you have float levels in the tanks.
Pressure is relative to temperature and demand.