I will tag @dennishoddy with this message since I know he does some pretty extensive RV'ing. Last year, we happened to be in Idaho and the wife's cousin needed a way to determine how much LPG he had in the LPG cylinders on his RV. I did some searching and found the following product, so we ordered one for him and tried it out on his tanks. After seeing how well it worked, I ordered one for ourselves, even though we have our RV hooked to a 250 gallon LP tank when we are living in our RV. (Still waiting for the work on it to be done so we can move back in.)
Here is a link to the Thuma LevelCheck. The way it works is that one places it against the cylinder at a 90 degree angle and presses on it. If it turns red, there isn't any LPG at that level of the cylinder. If it turns green, then there is LPG at that level. When we tested it on the cousin's cylinders, the outside temperatures were such that one could tell the difference by placing one's hand on the side of the cylinder and the gauge showed the LPG level at the same point. It isn't cheap (roughly $73), but it could save a person a trip to town or wherever to fill a partially filled tank.
Thuma LevelCheck
Here is a link to the Thuma LevelCheck. The way it works is that one places it against the cylinder at a 90 degree angle and presses on it. If it turns red, there isn't any LPG at that level of the cylinder. If it turns green, then there is LPG at that level. When we tested it on the cousin's cylinders, the outside temperatures were such that one could tell the difference by placing one's hand on the side of the cylinder and the gauge showed the LPG level at the same point. It isn't cheap (roughly $73), but it could save a person a trip to town or wherever to fill a partially filled tank.
Thuma LevelCheck