Find them on the horse trailer websites.
http://www.valleyvet.com/ct_detail.html?pgguid=59683dcb-1ed0-47c7-b732-d487536ef290Flat for bed of truck
http://www.horseloversoutlet.com/Water_Tanks_for_Truck_Trailer_s/60.htmreals slick compact one that fits in the corner that would work inside your fiver.
https://www.horseloversoutlet.com/Dry_Camp_Water_Caddy_30_Gallon_p/ftc-dc.htmsmall 30 gallon fits anywhere
I fill them right before I am CLOSE to my destination so I am not carring the extra weight for the whole trip and keep in mind in front of the axle if inside.