Here is an interesting remote indicator from Marshall. It works on their 254 regulator (the one used on many Airstreams) by sensing when the regulator has switched over to the reserve tank.
It is not a level sensor - simply tells when it is time to refill the primary tank.
It looks like it just snaps over the indicator window on the regulator. Maybe has an optical or hall effect sensor that watches for the button to pop up?
MARSHALL Remote Changeover Indicator - FSI