You can find tanks here:
Page 3 - RV Water Tanks
And then you can put it anywhere you want. I like underneath between the axels. Out of the way and no extra weight to one side.
Mine was right up front under the couch. Lot of weight on the tongue when full for traveling. especially when I bumped it up to a 40 gallon. Now I'm also putting a 26 gallon underneath, and will travel with water in that one, and fill the 40 when I get where i'm going. Thats if they don't have water hook up.