Do you need one - no.
But as others have indicated, there are many reasons to have one. If you wait until you are ready to sell, it will be an expense you're not going to want to bother with. But it will severely reduce buyer interest.
In addition, you never know when you might decide on a particular destination only to find out there are no hookups. Or a beeakdown, road closure, etc leaves you stuck with no hookup options. The fresh water tank is cheap insurance for that.
And, it will be a lot easier to add it back in now than it will several years down the road.