Compromise is key. Forcing them to unplug will be viewed as punishment, and they will never want to go camping again. The key is to plan activities like going swimming, hiking, etc that aren't conducive to using electronics. Also remember that smart phones/tablets are our cameras now, so they may want to take memorable pics when hiking.
So you as the adult plan the outdoor activities, and then when they have "on their own" time, if they want to use their gadgets, then let them.
So many weekends camping at the beach, our kids would be outside playing and swimming, but come around 4:00 they are looking for a break from the sun and the phone/tablet gives them a nice rest before dinner. Older folks may grab the newspaper or a magazine, it's the same with kids and their phones.
If it's a rainy day and everyone is stuck inside, we always had board games to play, but again, always gave the kids some of their own time to "plug in".
