I know sometimes the brochures are not 100% correct but according to them, the 40AD was a 2005 model. Maybe they extended into 2006 but didn't update the brochures.
We own an 06 Adventurer and looking to upgrade in the future. One reason is that we are concerned about the GVWR for full timing (We want to carry more than just the minimum stuff) and another reason is the basement AC. That 40 AD has a GREAT floor plan! We might look for a 40KD instead to get the king bed but that might not be such a big deal.
What does hold us back on the Winnebago line is the basement AC. I would ASSUME that if you found the need could add a roof on the Vectra line and not push the GVWR problems we are concerned about if we did that same with our Adventurer.
One other thing...get it weighed! I would consider stripping out all the stuff that might be in the basement and coach storage areas and verify it's current weight. Depending on the dealer you might convince them to fill the Water, LP and fuel tanks to do that. If not, you might need to do some mental calculations on what they would add. I don't think you would need 4 corners, just axle weights to verify how much cargo capacity you would have left. It would also help you know how close your rear axle is to the GAWR since most configurations appear to run out of that before front loading is maxed out. If you find you have lots of room to add weight to the rear area, GREAT! Given a lot of reserve, you will have lots of options on where you store stuff and if you needed to add an AC unit, you could handle that too.
These are just some of my ramblings based on what I THINK I know. Maybe I am over thinking our needs and projecting them to you. LOL YMMV.