What makes you think that is an unbelievable price? I'm not saying it's not (I haven't research similar makes/models), but make sure you're not being convinced by the
dealer's claim of how much below retail that trailer may be priced. That is only listed to make you
think you are such a lucky potential buyer.
Secondly, even at the current price make sure it's still something within your budget. And price aside, make sure this trailer's floorplan would work for your purposes. Definitely look at the rig in person too before you decide, as online photos can be deceiving in a number of ways.
That said, on the surface there is nothing wrong with buying unsold/older inventory from a dealer as long as it is discounted appropriately. In a numbers of months, 2020 models will be out and that 2018 will be 2 years old on paper... something to consider for depreciation, which is HUGE on RV's.