I've also heard good things about the Tank
Tender and am hoping to get my hands on one for my next
boat. I don't trust electronic senders anymore, especially the fuel level variety. But as long as it's working, no point in replacing it....
For the
propane, a squirt bottle of cold water works reasonably well- set it to a very fine mist, spray it at the tank, and the
condensation on the tank will reveal the fuel level. This works best if done immediately after using the gas system, when the tank is cold from the evaporating
propane. The only other ways I can think of are a scale under the tank (finicky) or a float gauge inside the tank (finicky, rare and expensive).