the problem is that propane in the tanks is a liquid, it evapourates in the tank as you are using it. so the pressure in the tank doesn't change as the propane is being used until its empty and too late
a mechanical level guage is the only reliable way, similar to a gas guage in your vehicle
the ones at Costco are the only rv sized tanks ive seen that have them and no, I don't use them. when a tank runs dry, switch over and fill up