Your friends have scared you for no reason. Dieting to give yourself constipation will make your trip more miserable. Bring with you wipes and toilet paper to wipe down the seat just as you would at any public restroom, and relax. Some restrooms are actually very well maintained by the camp hosts and are even cleaner then I have seen of some of my apartment clients.
One campground I stayed at the privy had no roof and only three walls the exposed wall allowed the throne to have a beautiful view of the valley and lake below and yet it was still private enough to take care of business.
