The length issue is a question of your level of comfort with towing. We have no trouble towing our 35' trailer long distances.
That said, I would be VERY concerned about the weights. We have the exact 'burb you are considering (8.1 with 4/1), and we have 2,100 pounds of available payload. We're a family of 5, with small children, and use almost the entire payload available to us. The 39' trailer will be heavier, your family will weigh more, and fulltiming means you will have more cargo. I don't see any way that you could manage that load within the ratings of any SUV, and I would not want to fulltime without being well within the ratings of my vehicle.
