According to Philip Lewis :

It would be much easier to put an airspace around an inner tank to
insulate the outside of the tank from the cold water, with the
condensate draining into the main bowl. (a double walled tank, kinda
like a vacuum flask.)

"Dewar flasking" it would certainly work, but it's overkill. An
insulating lining in the tank more than suffices in most cases.

[Isn't insulated toilet tanks standard down there? It certainly is
up here. I think you'd be hard-pressed to find any toilet up here,
short of an ultra-cheap model, that's not insulated.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.