What I do is weigh the tank and I have one of those strips that you stick in the side to guess at fill level.
Always check the connections with soapy water and a brush for leaks if you smell gas.
With two tanks the chances of running out of gas are slim. When one tank runs out you switch tanks. Your next tank exchange is usually waiting where you buy gasoline.
I wait until I am home an refill at my local dealer.
Yes you do have to unbolt and exchange or refill your tank.
As mentioned, you will hear the word cylinder used, which I believe is the correct technical term. Locally tank and cylinder are used interchangeably, except for small 1 lb. Units.