It may be a simple and inexpensive part to replace but I would STILL bring it to the attention of the Service Manager.
If you don't the thief wins and will move on to the next customer to steal from.
And there is always the possibility that if you brought it in with stuff in the tanks they most likely would have dumped both tanks "before working on it" and perhaps forgot to put it back. I have seen that policy in place in many service centers when working on any part of the RV. Just Saying.