I made a stainless one and the down side is with all the flexing one of the welds cracked. A metal tank will inflate a suprising amount when just being filled. Mine is mounted under the tray of the ute and you can hear the whole tray flex, and lift if I'm not paying attention to the hose, as the water goes in.
Next time I'd go plastic.
Team it up with a Shurflo diaphram pump with a pressure switch and bobs your uncle.
This will give you some ideas what is available
http://www.ovesco.com.au/products.php?catname=Water%20Tanks%20and%20Tool%20Boxes&cat=5&sub=21&subname=Water%20Tanks