mastiff:
got a relative that can't get out to check the tank and I can't drive to check it for them.
Propane companies do offer a service where they will keep the tank topped up. I'd guess they try to estimate when they need to come out based on past use so they don't make any more trips than necessary. It is an extra fee so over time would probably end up costing more than the sensor or whole system.
Of course that takes the fun out of the project but it's worth considering all the available options.