Why is she against the trip? If she doesn't like the high price of the trip, there are several clubs in the state that have trips/events there throughout the summer for a hell of a lot less money. I know my wife is very pennywise I wanted to go this year and you should have seen the look on her face when I told her the cost. I think she asked if the trip came with a free TV or something to that effect.
As far as selling it you could say that you want to take her on a little weekend getaway on beautiful Drummond Island (convieniently on the same weekend as the JJ). Forget to tell her about the jeep jamboree, and as long as you're there that weekend anyway, go on the the trail rides and join in the fun.
If that fails try to reserve a room at the drummond island resort and conference center. Drummond Island Resort and Conference Center- Drummond Island, Michigan
Show her pictures, we're planning on going there in April and the wife is more stoked about the resort than the wheeling go figure.