Heat the oil in a large cast iron skillet over medium heat on the grill (or stovetop). Add the beef and cook, stirring frequently, until browned, about 5 minutes.
Stir in the chili powder, ground cumin, garlic powder, and onion powder. Cook for 2 minutes, stirring frequently. Remove from the heat and season to taste with salt and pepper.
Scoop the ground meat into a bowl and wipe out the skillet with a paper towel.
Prepare the grill for indirect medium heat (350°F).
While the grill is preheating, assemble the nachos in the skillet. Create 2 or 3 layers of chips, refried beans, meat, and cheese, finishing with cheese on top.
Place the skillet on the cool side of the grill and cook until the chips, beans, and meat are hot, and the cheese is melted, about 15 minutes.
Remove from the grill and top with tomatoes and avocado. Serve immediately, with your favorite toppings on the side.