Both propane cylinders empty??
If so........small leak in hose fitting, connection etc. Large hole in hose would cause 'excess flow valve' inside the GREEN ACME nut connected to cylinder to close and next time WH tried to light it would fail and throw a 'fault' light (RED Light ON at switch)
Otherwise.........cylinders swapped on you full for empty by 'neighbors' is possible.
Buy couple of bottles of kids 'Bubbles'.......used in making soap bubbles
It's perfect soapy/glycerin solution for leak checking and cheap.
Pour in spray bottle.....use around all fittings starting at cylinders connections, regulator and then in/under trailer and at all appliances with full propane cylinder.
Most leaks that drain 2 full cylinders can (should) be smelt.
Hey 'neighbors'........who swapped my cylinders!!!