First, welcome to the Georgetown Forum.
As others have posted, it is quite common to have a motorhome chassis that is a year or two earlier than the motorhome model year. As the motorhome continues to age, the difference in the manufacturing dates will be of even less concern. Don't think of yourself as stupid, it sounds like you got a good price on a low mileage used motorhome. The original owner took a huge depreciation hit purchasing it brand new, and then had to deal with all the little problems that turn up in the first year. I did the same as you in purchasing a 2010 Georgetown (built on a 2009 chassis) in 2012.
Check over everything carefully at the dealer before taking delivery, and have any problems noted to be fixed. Here is a pre-delivery checklist you can use: RV Inspection Checklist