I'm a little confused, do you have two water faucets to connect to, or one faucet and a Y-connector? Are there 2 faucets and they are controlled by a single main valve? It kind of sounds like he made a mistake and turned on the black water rinse when he thought he was turning on the fresh water back on. Who told him to supply you with an anti-siphon device?
OTOH the valve was off when you got back and you turned it on causing the flood. Perhaps the maintenance guy planned on coming back to check his work before he turned the valve on, and that's why it was off. Unless he owns up to making a mistake it could boil down to him saying "I left it off on purpose, until I was finished". You caused the problem by not contacting him before you turned on the water. Sounds like a real mess, in more ways than one.
How full was the black tank? By that I mean how much "stuff" was in it. We've had a couple floods when the anti-siphon device in our black tank rinse failed. We've had no ill effects, but we were in a very dry climate at the time, and it was just regular water. I think you'd have a very hard time getting your insurance company to write this unit off as totaled.