Prepare your campfire or grill for medium heat cooking. You want a bed of hot coals rather than high, leaping flames.
In a large bowl, combine the diced chicken, cream of chicken soup, milk, thawed mixed vegetables, diced potatoes, thyme, garlic powder, and black pepper. Stir until well mixed.
Tear off four large sheets of heavy-duty aluminum foil (about 18 inches long each). Spray the center of each sheet with non-stick cooking spray.
Divide the chicken and vegetable mixture evenly among the four foil sheets, placing it in the center.
Place one or two refrigerated biscuits directly on top of the filling mixture in each packet.
Bring the long sides of the foil together over the food and fold them down to seal. Fold up the short ends to create a fully sealed packet, leaving a little room inside for heat circulation.
Place the foil packets on a grate over the campfire coals (or on a grill over medium heat). Cook for 25-30 minutes, rotating the packets occasionally to prevent hot spots.
Carefully open a packet to check that the chicken is cooked through (internal temp 165°F) and the biscuits are golden and cooked through. Serve immediately.