I always just used a regular Barker 15 gallon Blue tote with the little plastic wheels. It mostly dragged thru the sand behind the truck, but didn't hurt it. once you got off the beach and on the asphalt the wheels would roll again so you didn't tear up the tank.
If you really want the wheels to roll, get a regular blue tote and replace the stock plastic wheels with larger pneumatic wheels from Harbor Freight. Don't forget to air them down!
Get a couple of fresh water carriers as well, Wal-Mart has 6-gal Aqua-tainers pretty cheap. This way you can dump the gray and bring back more fresh with one trip.
