When I had a motorhome, it had a LP tank monitor. While not 100% accurate, it worked well enough. Unfortunately, travel trailers and 5th wheels don't often include LP monitors. To be fair, many people simply leave one tank turned off, and switch to it on when the other tank is emptied. This works well enough I suppose, but if you switch tanks in the middle of a camping trip, will you remember to fill the empty tank when you return home?
There are a few different solutions available for a tank monitor. One of the simplest is the "hot water" test. In this scenario, you pour fairly hot (not boiling) water over the tank, and the tank level can be determined by feeling the temperature change. This is kind of sloppy, and does take some time to perform.
Other solutions include a temperature gauge, however it suffers from having to be placed in the correct position, as well as you must be using the LP out of the tank at a sufficient rate to detect a temperature change.
I tried this option, and found it to be lacking. I bought the strips in the discount bin, and I confirmed why they were there... so save your money.
Finally, portable ultrasonic tank readers do exist, but they can be fairly expensive... as much as $70, which is a lot for such an instrument.
I was at a RV Rally recently, and a vendor was selling the Mopeka (AP Products) LP Tank Check system on discount, so I purchased one. This is a semi-portable system, and is Bluetooth wireless. The tank sensor (you can buy one or two depending on the number of tanks you have) attaches to the bottom of the LP tank via magnets and measures the tank level. You can use your smartphone (with Mopeka App) to monitor the tank level, or you can buy an optional dedicated montor.
So you can buy the Mopeka system in one of three versions:
I opted for the latter; the two tank version with dedicated monitor as I already have my smartphone doing plenty of Bluetooth chores.
The instructions are somewhat lacking, and I could not get the monitor to read the sensors until I called AP Products tech support line. I discovered that to make the monitor "learn" the sensors, you must depress the "Sync" button on the sensor 5~7 times to wake it up. They also suggested that using a bit of petroleum jelly on the sensor will aid in coupling to the tank. I didn't find that necessary though.
I also learned that the sensors use a common CR3023 battery, and the batteries last about a year. By all means, buy the batteries from Amazon, etc. as they are much cheaper than buying them at retail.
One thing I had to do for the project was to add a piece of King Starboard to the bottom of each tank as the bottom was open to the ground, and I was concerned the sensor might fall out.
The Smartphone app versions are supposed to have the ability to be calibrated to different size tanks. At least the transmitter batteries are shown, so if you have a smart phone, it might be a good alternative monitoring solution. However, there is a minimum OS that is requred for the app. For Android, it's Android 4.3 and up, and for an iPhone, it requires iOS 8.0 or later. Unfortunately, my phone is too old so I cannot use the smart phone app.