We have had a trailer or fifth wheel for more than 10 years now, and I have always been uncomfortable not knowing how much propane I have left in my tanks. I learnt early on to not open both and allow it to auto-switch over. I did buy a number of years ago a magnetic strip that you attach to the side of the tank and then pour water on (I think that was how it worked). I was not impressed.
This last weekend we camped and for Western Washington it got pretty cold. We were getting through the Propane. So I searched the web and found a product that seemed like it might help me.
These are magnetic and attach to the bottom of the tanks. I may add some duct tape too because I am nervous about losing them. They use an ultrasonic pulse/reflection to measure the level of gas in the tank. I tried it today and it worked well. It measured that one of my tanks was full (90%) and the other very low (15%).
The version I bought (for $72) is just 2 measuring devices, you read the level with a free app you install on your smart phone. They have another version that is a little more money with its only monitor display, that you could mount in your rig - its all Bluetooth btw - so no wires are necessary.
So far I am very happy with it and wanted to share with this group.