Online VIN decoders are notoriously poor, especially for RVs, and I have found a couple that say the VIN is invalid. But at least one RV VIN decoder
https://vinpit.com/vin-check/1T930BF1041074103
does identify the unit as a 2004 Trailmanor. That's good.
As for the diamond plate, TM is now putting diamond plate on the front wall of the front shell, as a stone guard. I think diamond plate might have been offered as an option during some earlier years, but I can't be sure. A romp through The Wayback Machine would tell you. I know that a few owners put on diamond plate themselves.
But there is no reason to have a stone guard on the rear of the trailer, no reason to wrap it around the sides, and no reason to have it on either of the lower walls, front or rear, since these walls are covered when the unit is folded down for transit. Also, the diamond plate covers the outside door to the rear storage compartment.
My guess - and it is only that - is that the diamond plate was added to cover damage when the unit was hit from behind, or possibly backed into a tree. It doesn't look objectionable in my opinion, and the fact that the trailer looks good inside is a plus. But if those rear walls were damaged, I would be concerned about leaks.
So my conclusion is that it is a 2004 Trailmanor. The triangular cabinets at the entry door say that it is either a 3023 or 3124. It is not a 3326 because that model had two axles. I think (again) that there was a kind of experimental 3225 model at one point - but not in 2004.
I'm not sure that any of this really helps unless the seller really opens up to you. He must have some other paper work - recent registration, for example. How about insurance documents? If he is in an accident after submitting bogus info to his insurance company, they will likely deny his claim. And they
will check all fields of the VIN, not just the 2 or 3 that I checked. By the way, the VIN appears on at least 3 places on the trailer - the one shown is simply the handiest and easiest to find. All we really know is that Trailmaster and T3 are bogus.
Bill