[sourdough]

Thanks for the explanation. Sounds like they need to come up with a plan if they are going to use the trailer much and Lynette had an excellent idea. When we dry camped all the time we had what she described; an external water pump (actually 2 - 1 12v/1 120v) to transfer water which worked out great vs trying to dump water containers into the fill port.

As far as what's in it now, I guess you have to hope it's just something that's not too bad. From what I've seen non potable water can be most anything.
Even if they're "going to do what they're going to do" they need to drain the tank. Since acquiring fresh water is an issue maybe they can come up with a way to get at least 1/4 - 1/3 tank filled then add a double dose of chlorine to the water, pump it through all the lines, let it sit a minimum of 4 hours then dump. Most anything is going to be better than nothing. I wouldn't worry about a chlorine smell when all is said and done. The water system where we live routinely has a strong chlorine smell and we use it; we DO use an RO system for drinking water though.