Comparing Jayco X18D to Roo/Shamrock 183 and Starcraft 187TB

They are all so darn close in quality it really makes no difference which you go with, you can still get a lemon with any of them. Just pick out the floorplan and color scheme you like best. I had heard somewhere that something like 80% of people trade them in in the first 2 years anyway which the folks selling the 2012 you're looking at discovered.
You can go to RVdirect and see what a new Jayco should go for, compare it to the used one. Sometimes driving to get one of these direct sales units are worth the drive.