Weigh each tank... 18 pounds is empty, 38 pounds is full.
Snoop the lines for leaks(not likely)
Check the screen on your regulator to be sure it is clear of obstructions. like ice in cold weather, or dirt, spider webbing, or wasp nest.
If it is plugged it will gradually reduce pressure to zero(yellow is part way there!)This is most likely.
If it is clogged with corrosion the result is the same, except that you will need a new regulator.
BTW, I'm assuming that you do not have one of those useless in line fuel gauges, If so remove it.