The 59-year-old Grand Rapids man was later transferred by Aero Med helicopter to Spectrum Butterworth Hospital in Grand Rapids.
Emergency crews responded to the campground at the Grand Haven State Park at about 2:30 p.m. Saturday on a report of a man trapped under his camper.
Police said the man was trying to move his fifth-wheel when the blocks holding up the camper fell out and the tongue of the trailer struck him.
People in the campground helped free the man, who was then taken to North Ottawa Community Hospital.
The man remained in serious condition Sunday evening, police said.