Keep in mind the size of the show. A medium size show (>50 sellers) and large shows (>100 sellers), you will have a better chance at getting what you want. At a large show, dealers will buy the junk silver and turn around and flip it by acquiring more inventory from another dealer or outright selling it to another dealer for a small profit margin. They don't want to tie up their buying funds in junk silver only to not be able to buy the coins of some old timer toting the ratty shoebox of coins with more intrinsic numismatic value. At small shows, buyers won't be inclined to pay top dollar as they know the competition on the bourse floor is small.
Your best bet is to bring only a handful of rolls (5 or so). Leave the rest locked up in the car out of sight. Scout out the bourse floor and identify the larger sellers. Also, look at what they are selling. Dealers who deal in silver bars, rounds, and a steady business of bullion are more likely to give you a price closer to spot. A dealer who specializes in early coins will have no interest in acquiring junk silver even if they can flip it for a small profit. It just isn't worth the hassle to them and isn't worth the hassle for you to waste time asking. When you identify a few dealers who are close to your expectations, approach each of them and ask if the buying price will be greater if you sell more. Get you best price, and sell.
Good luck.