Check with another dealer that has the ability to work on box trucks. There is no reason the camper must come off for the truck to be serviced except that they don't have a way to pick it up due to the clearance. The place that can work on box trucks will either have a pit they can pull over or are just willing to crawl under the vehicle.
However, if you do just want to remove it, there is nothing you need to be worried about except making sure the truck doesn't catch on anything. Trick is to take it real easy and stop the second you notice the camper has caught on something.