I purchased one of the level gauges gasman posted the picture of. I had the local propane/BBQ company order it. I have it installed on the door side tank. When the other tank runs out, I can closely monitor the door side tank to give me lots of heads up time to get more propane. It seems to very accurate.
Tom & Renée
2015 Wildcat 295RSX
2012 Ford F-250, 4x4, 6.7 Liter Diesel