Do you need to sanitize a water tank that's never been used?

If it makes you feel better go for it but since you say it's "never been used once", I'm assuming a brand new trailer and it really isn't needed.

I might fill and then pull the drain plug in case any dust go inside but unless you have reason to believe it was contaminated, I really wouldn't worry about it.

We've also never bothered to sanitize our fresh water tank...and WE DO DRINK FROM IT. Been doing it 4-8months of travel per year for the last 12yrs and on vacations for the last 50yrs. Never had any issues.