I guess one of the lessons to be learned from this is to look at the tank monitors during the pre delivery inspection.
The only thing that can possibly be done is complain to the seller and hope that he makes it right somehow. He does not "have" to, since it was a used unit and some un-named minion must have forgotten, and this is the first time this had ever happened, ya de ya de ya.
Meanwhile you have a stink you have to fix, and dumping the tanks is the only solution. That is a significant and necessary part of the rv-ing experience.