Honestly, I have never had to purge because my tank ran out and I refilled it same day. I've had to purge when I shut my tank off in November and didn't turn it on again until March.
You should be able to valve your tanks shut, and not see the green in your indicator turn red for a couple days (assuming you turn off all the gas consuming devices). Anything faster than that, and you should be looking for leaks.
In your case, the furnace and WH may have been trying to pull gas, which would turn the indicator red, but still would leave low-pressure propane in the lines and not require a purge (air requires a purge).
If your heater runs for five minutes without really heating, it has a problem.
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2019 Cougar 26RBSWE
2019 Ford F-250
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