INSIST that they hook it up, put out all the slides, and turn the a/c on in the afternoon when it's nice and hot in there......lots of a/c problems in the time frame when we bought ours in May 2005, serial #455.....our dealer alone had 3 Big Horns with a/c not working correctly. Also check cabinet attachment points if you there are free standing ones, & if it has the remote control unit for a/c, have them replace it with a wall mounted one; check to see that dump valves are labeled correctly, all units without exception had labels wrong in that time frame, check for fresh water drain; & if it has a fireplace, make sure it is not mounted directly on the carpet.....it won't work properly, has to have little feet under it to get ventilation flow....
Must be other things but those are all the ones we had initial problems with, and since it's an early 2006, these issues were there on quite a few units.
Good luck, you should get a good deal on that unit.
Pat & Logan Doyle
Big Horn 3400RL #455