Dual pane windows were a prerequisite with the purchase of our 3410. We do a lot of cool weather camping so I was looking for a better sealed and insulated trailer wanting to avoid any lack of heat and condensation issues. It's better insulated for hot weather and keeps outside noises down as well. But yes the will add weight, how much, I'm not exactly sure. I was told to add 1000 lbs, but I don't think it's that much, maybe more like 500 lbs or somewhere in between. Frankly I don't care what they weigh because the living quality goes up substantially with having them!
I'm sure the dealer would be more than happy to install them after but it would be very, very expensive! The windows would have to be ordered and shipped from the factory, the old ones taken out and the new ones installed. The rear window must be worth at least $400 plus a few hrs of labour alone!
Save yourself the time and money and order them with the coach. You won't regret it, but you will have regents if you don't!