One of the tanks not turned off tightly......
Then leak could be from around stem of packing gland on service valve. Service valves have double seats.....fully closes & fully open. In between packing seals the stem.
Course with an 18 yr old rig........
Pigtails between cylinders and regulator
Regulator itself
Feeder hose from regulator to distribution pipe
Any fitting on dist. pipe
Any supply line from dist pipe to an appliance
Any appliance gas valve
Go to any toy store/dollar general etc. and buy bottle of 'Bubbles' (kids use to blow bubbles with)
Spray bottle and couple cap fulls of bubbles.....valve in the cylinder that was left partially open and start spraying at cylinder working downstream.
Tiny bubble, in your barn....tiny bubbles makes you learn :B