On our first trip to the fort we were invited by friends, who were going to be there for a week after we left. Once we got back home, I sent them a thank you package through the USPS. I called the fort, and they gave me their mailing address, and said that people regularly receive mail there.
I sent it to:
Fort Wilderness -
Kaiser Family #????
and the fort address
My friends never got the package. They had received notices on their vehicle, but didn't know what they were for, and was really having too much fun to go check to see what they wanted.
The package was returned to me like 2 weeks later.
As long as the person your sending it to follows up on their notices, it shouldn't be a problem.