Joe I think there is something wrong with that trailer. You don't need hazmat placards for food or water trailers, except maybe high proof alcohal, so I would never use something that ever had those kinds of labels on it for water storage. It's also only rated for 25psi so your still going to need a proper pump and pressure system.
I will say the nice thing about a water trailer is you could tow it to a water source, lake or river, and fill it. It would still require filtration after you got it home but it would be doable and something that sized is going to need a rig to tow it with.
You could probably get a smaller water trailer, say 500 gallons, and use that to transport water from your source to a underground storage tank. Obviously your going to want several layers of filtration.
Some people will do skanky things for $25, and not all those people are crack whores.
San Jose, CA