The caravan industry is notoriously unregulated; so the answer is no one. Have a look at discussions on caravan warranties on this forum and what lemons they can be in relation to caravans.
How did they get it from the rainwater tank to the caravan? How long was it sitting in the caravan? When was the last time the caravan’s water tank was cleaned?
I’ts quite possible algae grew in the caravan’s water system, especially if the caravan was nice and hot from being in the sun.
