Truma do a Bluetooth gas level checker, says it uses ultrasound, I wonder how easy that would be to replicate.
https://www.truma.com/uk/en/products/truma-caravan-rv-gas-fittings/truma-levelcontrol.html
Can’t you place it on a smart scale? And measure the weight?
you can get a weight sensor and attach to a esp8266 nodemcu.