First I found what trailer I wanted, quality and function, floor-plan, etc. Then I watched the 5th wheel trailer prices for 5 years. I had to wait to retire before I could buy one. On my specific trailer, I found that dealer would be willing to negotiate one year after new model was introduced. Sometimes the next year model was introduced in first part of current year. For example, 2017 could be introduced as early as January 2016, so the 2017 model would be around for 2 years before it became dated. The longer you can hold out before buying the latest model, the better off you are. I believe the dealer gets to place a trailer into their lot interest free the first year, following year dealer has to pay interest. Second year on the lot, cost them money, so they are willing to move older models. January and February with snow on the ground seems to be a better buying month. You also get more attention during your PDI. There is usually a “born on date” on the trailers. Somewhere, the date is noted when it was build. That is the most important date. My dealer usually got delivery on the trailer the same month it was built. Try to find two or three dealers that sell the same trailer and then work the system to get the best deal.
I also believe that unlike cars, MSRP is a made up number. Dealer adds or has factory install a bunch of options to the trailer and inflates the price. I have no hard facts, but I believe that the markup on the trailer is between 35 and 45%. Find a high volume dealer near you. Depreciation, funny thing, it varies based on location, and the kelly book did not help me much.
On a separate note, I found that most people try to sell their used trailer that may be only one or two years old for more money than you can buy a new one. Most likely due to the interest they paid that they are trying to salvage. They can't sell it, and give up and trade it in.
And lastly, unless it is a major issue, plan on fixing things yourself. Make sure everything works when you do PDI.