Something else to consider, for those who may not be aware is, places like gas stations, grocery stores, etc., where there are "swappable" tanks, you give an empty and take a full one, are NOTORIOUS for short filling their tanks. They say it is for safety, but there is a blow off valve for that.
I would MUCH rather have a tank that I owned, whether BBQ style or DOT approved and fixed, that I then had filled by weight or volume so I knew exactly what I was getting and how much I was paying. Here is a article to explain. I have weighed my BBQ tanks and they were all short on LPG.
https://www.orangecoat.com/the-truth-about-filling-20-lb-bbq-grill-propane-tanks
The BBQ fill up is a bit deceiving because they first hand you a slip that says "20 lb cylinder". When you pay inside the receipt says "20lb PROPANE", and the filling print out says "Cylinder: 20 lbs." The only defense is that the filling print out is honest and says "3.6 gallons". However, nobody knows off the top of their head that a propane tank is supposed to have 4.7 gallons to be considered "full". By saying 3.6 gallons, they are masking the fact that they put in 75%. If they wanted to be upfront they'd say "we will put 15 lbs of propane into this 20 lb cylinder".