If going the antifreeze in lines route, 2 gallons is about right as Jamie said... drawing it thru the lines at each fixture already gets some in the tank...
If blowing out lines instead (compressed air), still need to put antifreeze down each drain to protect traps and tanks... but a gallon would be enough there...
Freezing is a problem where there is no room for expansion... water supply lines, p-traps... the dump tanks when virtually empty have lots of room for expansion... want enough in there to get into the drain pipe to dump valve, but that’s all that’s needed there.
Of course moving to FL makes the whole process go away...
If you have a rig with the additional dump valve (typically when tank is opposite side of RV from dump valve) - be sure that valve is closed, and line running under the RV is empty...
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