Water Tank Replacement/Addition

I'd like to increase the capacity of my fresh water tank. There is room under the bed to either add a second tank, or replace the current tank with a larger one. Ideally I'd like to have around 40 gallons of capacity. I don't travel with water in the tank, or at most just a couple gallons, so weight isn't a problem.

Can anyone recommend a quality tank supplier?
Has anyone done this in a T1950?

We camp a lot a state parks where there aren't water connections, but we can fill the tank when we arrive.

Thanks for any help.:idea: