I decide what I want to pay and what condition I'm going to accept (which is generally 'mint' or 'pristine'), and when I see a camper in the condition I want, at a price I'm willing to pay, I am on my way with cash in hand. This applies to both new and used items. Even paying a premium for a used camper in excellent condition is generally still a huge savings over a new one, but I've also purchased several new "last year" campers at a $10k+ savings (they don't last long when that clearance tag goes up- you snooze, you lose). You'll certainly quickly get a feel for what campers of a particular age and condition are selling for in your area- the overpriced ones stick around forever, and the ones that disappear the next day were probably priced below the "fair market value" in your area. Naturally, rarity of a particular model, special features, etc., may increase the going price significantly beyond what a linear depreciation estimate might yield.
When I sell items (including campers), I regularly get tire-kickers who lament the fact that they've been looking for XYZ for years, without luck, and make a lowball offer. As I decline their offer, I can't help but thinking they'll be dead and buried before they get that Cadillac at a Kia price.