The best place to find RV and Truck camping water tanks is off a horse trailer website!
They make great corner fit water tanks that fit real nice in a toy hauler and inside an RV. And real nice flat ones for the back of a truck.
This is the one I use in my horse trailer and then take it out and put it in my MH when RV'ing when I will need additional water once I arrive at my destination. I fill it up when I get there before parking the coach. Will hold 25 gallons. The best feature is where the water comes out so you don't have to use a hose.
http://www.horseloversoutlet.com/25_Gallon_Slant_Load_Stand_p/ftc-slst.htm To see all of the different ones available go here and scroll down to bottom.
http://www.horseloversoutlet.com/Water_Tanks_for_Truck_Trailer_s/60.htm