Welcome to the forum.
Couple of places you can check frame problems would be the Dutchmen web site under the customer servive tab Recalls and the NTSB.
Frame issues are usually identified fairly quickly if there has been a defective batch and the fix is usually part of a recall. If you have the VIN You can check for open recalls.
The last frame problem I heard about was cross brace issue that was part of a recall, unfortunately I don't remember the model.
As you are looking at a 9 year old unit, I recommend you invest in a moisture meter and check the exterior walls and the floor for any signs of moisture. Most big box home improvement stores have some relatively inexpensive pinless moisture meters. If you detect any moisture, run do not walk away from the deal. RVs are like great big mould incubators if they have had any leaks and they haven't been tended to properly.
Just take you time looking at the trailer make sure everything works and if buying from a dealer make sure everything is in writing.
When I was a kid I was going nowhere fast, now I'm an old man I'm going everywhere slow.