The problem is not water in the tank. As stated, a couple of gallons in a black or gray tank won't cover the bottom and if it freezes, it won't do any damage. That's not true with where the water drains "from the tank".....
If you leave the valves open and have the cap on the sewer connection, that area will fill with water and if it freezes it may well crack the pipes. Same with any plumbing between the tank and the valve....
Adding "a gallon or two" of RV antifreeze to an "unknown quantity of stuff left in the tank" is a sure fire way to destroy a valve or crack a plumbing run, especially in any area where the temperature falls below freezing for an extended time.....
So, I'd strongly recommend emptying the tank completely, not taking a risk with residual liquid migrating into the plumbing run to the valve, freezing and damaging the plumbing or the valve. Pouring a gallon or tow of antifreeze into a tank with one gallon of liquid is significantly different than pouring a gallon or two into a tank with 5 gallons of residual liquid. One "might be OK down to 20F while the other is going to freeze at 20F, possibly cracking that 3" pipe that connects to the black tank, just ahead of the black tank valve.....
You know it's going to get cold enough to cause damage to unprotected "stuff", so I'd take action to protect the "stuff" knowing it's potentially at risk..... YMMV
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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