maybe try and change your strategy? I would imagine many of these dealers get 100's of inquiries for every sale.
How about a phone call to a live person first to narrow down the rigs. Once you finalize on a few, than revert to emails.
Or if you are stuck on only emails, make the first email very short with 1 or 2 questions only. Questions like "best price", mileage...easy things like that. After you get a reply, than a phone call to that salesman might help too. Let him know you are a serious buyer and not just a lookie lou.