We carry bathroom scales. The full weight of each tank is stamped on it somewhere (usually the protector/handle ring at the top). If a tank feels like it's getting low and I wan't to know exactly, I just put it on the bathroom scales and weigh it. I subtract the actual weight from the stamped full weight and that's how many pounds of propane have been used. Propane weighs 4.2 pounds per gallon, so with my 30 lb. tanks, that's 7.14 gallons when full. I have one of those Dometic gauges linked above, but have found that I have to pull the tanks to get an accurate reading with it, anyway - so the scales are more accurate.
Rob