I have the water tank that Darrin mentioned on my trailer, only problem with mine is that theres a stupid little hose that you have to use and to fill it you need a very long hose that goes from running water over to your parked trailer. (unless the water tank comes off so correct me if i'm wrong! )
I use the empty Culligan water containers. You pop the lid off and fill them. You can also buy these for about 2.00$. This way your water is portable. It is great.