I have good luck just looking at the tanks in the cool early mornings... You can usually see the wet line of the fuel inside the tanks... I can also just feel the tank in the mpornings and can tell where the liquid line is inside...
I added a couple of those inline gauges one-time and found they always read either good or bad... Never shows anything in-between haha...
Someone sold a strip thing you could stick on the side of the tanks and maybe pour a cup of hot water on them and can see the level of gas in them. I used this for awhile and it worked pretty good. Then I changed tanks and it got exchanged at the propane guy's place haha...
I have one of those small D-RING's clips I hang on the active propane tank to tell me which one is being used...
Roy Ken