Christopher and Roberta Laundrie are really evil. You know the first thing they asked Brian when he arrived home is where is your fiancee. They know he murdered her and are hiding his whereabouts from the police. They immediately lawyered up and refused to let him talk to the the authorities. They refuse to answer any questions about Gabby and what happened to her. They know where he is right now and helped to arrange it. They know well that he is no place in or near the swamp. There is a reason for the 2-3 day trips. Where Brian is now is related to those 2 trips to Fort De Soto Campground somehow. The FBI should be checking out who was camping there during those 2 trips and how they are associated with Christopher Laundrie for example his neighbor Vincent Doria for one.
My spidey sense tells me only one thing, they know he's alive. Otherwise they would not be carrying on. getting doordash, mowing lawns, playing games with media. They'd be heartbroken and telling the police everything.