After much research here on the site and a months long search, I was very excited to purchase a very clean 2004 Trailmanor 3023 from a private owner. She told me she was the 2nd owner, and the original owner kept it in their garage and didn't really use it that much. Everything checked out and worked on the trailer, and though I couldn't see the date code, the tires were properly inflated Goodyear Marathons, that looked to have 80% of their tread left.
So before we took our first "real" camping trip in the next week I scheduled an appointment with my mechanic. He does not deal with trailers much, but was willing to take a look at the tires and brakes since I was concerned about them.
His associate went ahead and took one of tires off, and we found the following date code: "0804".
Yes, apparently, the trailer had the ORIGINAL tires still on it!
Needless to say, I have 3 Maxxis ST225/75R15 Load E on order.