We deal with this almost everyday at our motorcycle consignment dealership. Our rules are the motorcycles have to be in street legal condition, as stated by the manufacturer, in order for us to consign the motorcycle. Motorcycle tires should replaced after 5 years or when visual inspection finds signs of trouble. The date we go by is whats stamped on the tire, not when it was mounted on the motorcycle. When we order tires we will not accept any tire over 18 months old. My research on my coach seems to indicated that RV tires have a 5-7 year life...depending on how they are used, maintained and stored.
Recently a customer beat our price on line...they shipped him 2012 tires and we refused to mount the tire...liability. We ordered from same distributor and we received 2017 tires. Not all, but most "Deals On-Line" are being sold from: stock-outs, dealer liquidation, close-outs, etc.
Remember, in most cases, when you start to see dry-rot on the outside of a tire, the deterioration inside the tire will be far worse.
Not only did your dealer sell you old tires, but they now have opened their business up to possible major liability issues in the future.