Francis Fisher plays Jonathan Taylor Thomas's character's mother in this film. In 1991 she was cast as his character's mother in the show "Home Improvement", but was replaced due to poor early audience reaction to her.
When the Dad crashes his big rig truck, the shot inside the cab reveals a woman to his right with a lit cigarette in her hand. She grabs him on the right shoulder and then he swerves to the left. Don't know if she is giving him a cue to swerve or she is the "hidden" girlfriend. We never see her again though!
The scene where Marshall and Mark are in the swamp, Marty is clearly seen not filming them being chased by the alligator but later on in the movie, when they show their film they show the alligator chasing Mark and Marshall.