I would think a lot depends on the RV. My water systems are enclosed in an insulated compartment. If you have exposed tanks and/or plumbing your situation would be far different then mine.
I don't worry if the night time temperatures stay above the low 20s IF day time temperatures get into the 40s
I do get worried if night time and day time temps stay below 30.
A week or so ago, I saw night time temps forecasted for 22 with a day time in the high 30s so I turned on system heat (water compartment heater) and hot water heater.
This past weekend they forcasted lows of 18 I winterized.
Although water starts to freeze at 32 it will take FOREVER, remember your refrigerator freezer is probably set for zero or less and it takes a couple hours to freeze ice cubes.
The heat soak from daytime high has to be lost before freezing can start.
JMHO