Anyone can visit Fort Wilderness and its campfire program (which includes the outdoor movie) for free. I believe that you are technically supposed to be staying at a Disney resort hotel to attend the campfire program, but there is no sign indicating that at the campfire area. It is an open area and no one checks to see if you are a Disney resort guest. They probably don't check because the venue is rarely full (it's all outdoors and there is lots of seating).
If you are driving to Fort Wilderness and coming in through the Magic Kingdom parking gate (most hotels near the Magic Kingdom are approached by first passing through this gate), you would have to pay a fee for parking in the Magic Kingdom parking lot, but maybe the gate attendant would wave that fee if you say you are going to the Fort Wilderness campfire program.
Or you can drive north along the road past the Port Orleans resorts, then turn left when that road ends, and 200 yards down that road you will find an entrance for the parking lot for visitors to Fort Wilderness.
Note that if you try to enter the main entrance of Fort Wilderness, unless you are a guest there they will direct you down the road to the visitor parking lot (the visitor parking lot is surrounded by wooden posts so you can park free for a visit, but you can't drive all the way into the campground through the visitors lot).
Then you walk over to the internal campground bus stop and take the Fort Wilderness bus to the campfire program (you may want to ask the bus driver to mention which stop is for the campfire program).
I highly recommend the campfire program and the movie that they show afterwords on the big outdoor movie screen. It is a Disney experience that everyone should have, and it is FREE!