I have a cheap magnetic tank indicator tape that you can get just about anywhere. By sticking it to the side of the tank and then pouring hot water over it will change color to tell you the level of propane in the tank.
I only open one valve on the top of my tanks at a time.
I use the tanks as a Primary and a Secondary. When the Primary gets empty I open the Secondary if I need to get through the night or something. Then as soon as possible I refill the primary and then close the secondary tank and open the primary. In this way I'm almost always running off of the first tank and hardly ever the second. The second tanks last for years this way.
By using my magnetic level indicator I keep a eye on the propane amount in the secondary tank. When it gets to half empty I move it to my home gas grill the next time the grill tank runs empty. Then I purchase and put the full tank back on the RV in the secondary spot.
I hope you understand what I mean I may not have explained that very well.