Pennsic is like a small town. With over 10,000 residents, it is a small town. Like any village, some services are available and some aren't. The Cooper's Lake staff does a very good job of providing the basic necessities for their guests. For instance, there is a camp store. It is small and doesn't have a huge selection of items, but it can meet basic needs. The store sells milk, soda, limited supplies such as bread and fruit, laundry soap in small quantities, matches, batteries, candy, and ice cream. It also provides cubed or block ice for $1.00 per bag and firewood by the truckload. Don't expect to buy all your needs here, but don't be surprised if the store can get you through in a pinch.
The Coopers also have a small laundromat on site. There are only three washers and three dryers available, but it is open 24 hours a day and standing in line can be a great way to meet new friends. In addition, there is also a small post office where you can buy stamps, postcards and the like, but the facility's open hours are limited. A small bank that can cash travelers checks and make change is also available in the merchant's area, but it does not cash personal checks. There is now an ATM on site, but its hours are restricted and lines tend to be long when it is available. Telephones are located near the camp store and usually have a line during the day. It's often easier to make phone calls in the evening. There are several cell phone towers in the vicinity, so some cell calls are now possible.
Now, for the question you were afraid to ask: Yes, there is plumbing at Pennsic. It's just in limited supply. The camp has a huge number of port-a-johns (port-a-castles) available. They are remarkably clean and well-maintained considering the number of gentles on site. There are also indoor bathrooms located in the bathhouses. Cooper's Lake has three bathhouses, two hot water and one solar-heated. (The solar shower seems to have as much hot water as the others and is much bigger. Less waiting!) Plan to walk a ways for a shower unless you have your own camp shower facility. Water spigots are located very near every camp. The water in the past has been terrible for drinking. It is perfectly safe and clean, but has a very content of iron and other minerals. If you are sensitive, you will want to bring bottled supplies.