Why didn't they check for campers?

Because the camp grounds were on the other side of the river from where the dam was.

The camp grounds were not part of the intended strike zone, the camp grounds were across the river from the dam.

But the camp grounds were flooded as a direct result of the dam's destruction. When the camp grounds were flooded, the man, who was sleeping, drowned, and his body was carried far away from the area by the flood, resulting in a lengthy search to find him.

One of the film's key themes - it didn't matter how meticulously they checked the area around the dam, it didn't matter if they had access to live streaming footage proving nobody but themselves was in sight, what mattered was they were trying to solve one big problem with one big solution, and the documentary film maker at the beginning of the film cautioned Dena that the only concrete way to solve big problems was by starting with small solutions.

Another key film theme - it didn't matter if they checked the area for hours to ensure nobody was there, what matters was, bombs by default cause damage no matter what, and damage is unpredictable (think about drone strikes, missile strikes) and damage is far-reaching on many levels - damage is physical, psychological, spiritual, damage strikes individually and strikes society. They thought the damage would be restricted to the dam itself, but the damage was far-reaching (forest fire, fire killing all types of animals, amphibians, reptiles, birds, mammals, etc, floodwater destroying ecosystems, firefighters putting their lives on the lines, etc), the damage took a human life, the damage unsettled an entire town, the damage unsettled the organic farming cooperatives, the damage destroyed each other, the damage was much more intensive than what they predicted because they tried to solve a big problem with a big solution, something that never works out the way you calculate it will work out. They thought their big bomb would bring national attention to an important environmental issue and restore their salmon fishing industry, and they thought those two results would magically solve any and all problems resulting from the big solution, and as the director at the beginning said, there is no one big solution.

The guy wasn't in the campground, he was on the riverbank where people don't normally camp, and they didn't have time to scout the entire area (and even if they did, they would be maximizing their chances of being seen by as many witnesses as possible).

 Sig under construction 