I have one of Plastimo water bladders that I used in bus conversion and saved for future use on my TC.
But that brings several issues that you need to evaluate.
We had that discussion in other thread, where a member had plastic square tank on roof of his TC and garden hose attached to it for using around campfire and such.
Would I want to use the setup on close to me desert, the water would be about 160F in the afternoon.
Than I don't think I would be comfortable having more than 300lb of water on the roof, still assuming the weight is spread.
I am trying to make a set having extended stay at Powell Lake in mind.
So far the idea is that I drive TC with boat behind there, detach the camper, launch the boat and then use truck, or boat trailer to bring 60 gallons of water in the bladder. Electric pump is easy and I can either use transfer pump to fill up TC tank, or hook up pump from the bladder to TC directly.
I already bought high-volume 12V transfer pump at HF for less than $40 and probably will skip the idea of getting RV pump for direct connection as those are expensive and prone to more headaches.
Than staying on the beach - I use solar shower bladders for outside use, what lot of time is more convenient and saves you trips with gray water tote.