1. If assembling at the campsite: In a bowl, combine graham cracker crumbs, melted coconut oil, and maple syrup. Stir until the crumbs are evenly moistened and hold together when pressed.
2. Press the crumb mixture firmly into the bottom and slightly up the sides of an 8- or 9-inch tart pan (or disposable pie pan). Use the bottom of a cup to press it flat and even. Place in the cooler for 15 minutes to set.
3. In a separate bowl, mix the Greek yogurt, powdered sugar, and vanilla until smooth. Taste and adjust sweetness if needed. Spread this filling evenly over the chilled crust.
4. Arrange the fresh berries on top of the yogurt layer in a single layer, pressing them in slightly. For a shiny finish, warm the fruit jam with a teaspoon of water and brush it over the berries.
5. Refrigerate the tart in the cooler for at least 1 hour before serving (2 hours is better). Slice and enjoy immediately. Store leftovers in the cooler for up to 2 days.