You will need:
Somethign like this (link to Amazon.com)Put it in the propane line on the active tank (Turn off tank before removing line) and then turn on tank, with EVERYTHING OFF (stove, furnace, oven, water heater) turn on the tank, wait one minute and turn it off, note gauge reading.
Come back in an hour and note reading
Come back tomorrow and note reading
If the gauge varies a bit.. That is differences in the ambient temp.
if it drops a lot, that is a leak.
If it remains in the green overnight.. No leak.
Instructions provided with gauge
NOTE: some RV's have said gauge already installed.
100 PSI is in the green, 50 is not.