IIRC, the manufacturer is required to put the date of manufacture on a label (not just a sticker, but, a long life label) adhered either outside on the street side of the trailer or inside (may be in a cabinet) near the main entrance door.
Seems to me the '08 210 WBS that we used to have, had the sticker on the outside, along with the tire size and pressure information. The Cougar 318 that we have now, also has it on the outside just above the label for the tire size and pressure. Our Cougar was built 2/10/11 (one of the last of the '11 models rolling off the assembly line).
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)