Do we have to winterize if we use our RV during the winter?

You winterize for that one time when it freezes enough/ long enough for pipes to freeze, so doesnt really matter one day or 90 days, but, BUT....

Some people never winterize and are fine when pipe freeze solid, yet some have pipes burst.
The PEX pipes do much better than any other pipe.
Plastic water tanks take a very long time to freeze and are able to expand some, I have seen people store a full fresh tank for years in freezing winters.

Up to you, there are risk no matter what advice you get...