remember, the moment you sign the papers, it is a year old used item. The market does not care that you have not driven or used it yet. The dealers will try and tell you that you have a new product that will not depreciate any more than a similar 2018 today, or at best offer a marginal compensation. You will have to beat reality into them, their current coach should depreciate similarly to units they sold 6 months ago and have been sitting in the owner's winter storage.
In the summer of 2019, you will have a 3 year old coach if you decide to sell (2020s will be all the rage). Flip it around, if you were a used coach buyer, would you value a 2018 more than a 2017 at that time. You need to capture that difference at the time of your purchase because you will likely loose it at the time you decided to sell. The longer you keep it the less the 1 year difference will amount to when you eventually sell.