Those gauges that screw in the valve are pretty much useless. The vapor pressure in the tank is pretty much constant from full to empty and more dependent on temperature than anything.
The most accurate gauges measure the liquid level in the tank. Some are a thermally sensitive strip that adheres to the side of the tank and show liquid level when you are using the tank because the liquid will be colder from evaporation. The best one (IMO) is the Manchester tank “Sure Flame” 20 lb tank with built in liquid level gauge. It is a little non-linear (like a car’s fuel gauge) but it works great.
Gene – as far as U-Haul for fuel, I don’t think the U-Haul places around here sell propane but I’ll double check. It would be nice to just pay for what they put in. This way, I can top-off my cylinders instead of running them empty.
Joe BBQ, who fills his Weber grill tank every year won’t really miss 5 lbs of fuel. But as a vending cart owner, I know that a full tank will run the rig for 5 days. If I’m down 5 pounds of fuel, it impacts my business.