Are we discussing Propane Portable CYLINDERS for Trailers (DOT Jurisdiction)
OR
Propane Fixed TANK for Motorhomes (ASME Jurisdiction)
Tanks ....typically have gauge on tank and can have remote sender to inside panel
Cylinders ... there are 'gauges' that connect between OPD Valve and Pigtail (limited info cause they indicate pressure)
There are heat sensitive tape you stick to outside wall of cylinder
There are weight sensors that you place under the cylinder and then they transmit weight to receiver/app on phone etc
And then one can just lift the cylinder to guesstimate how much is left
OR use a auto swap reg (if you have 2 cylinders) ....1st one runs out RED Indicator shows (flip indicator level...GREEN shows) remove empty cylinder and get it refilled
*REPEAT as necessary