Buying a New 2016 trailer in 2019. What’s it worth?

There's no point in even having this conversation. A brand new Grand Design Reflection 303RLS (very similar floor plan) has a "sale" price of $46K at numerous dealerships. Taking just 20% off of that number gets you under $40K. There are probably countless other manufacturers with nearly identical floor plans for around the same price. I'd make the dealer an offer of no more than $20K and walk away if they don't accept it.