Preheat the oven to 375 degrees F (190 degrees C).
Bring a large pot of lightly salted water to a boil. Add manicotti shells and cook for 8 to 10 minutes, until tender but not mushy. Drain.
In a medium bowl, stir together the ricotta cheese, crab, parsley and onion. Spoon into the manicotti shells. Place in a buttered 11x7 inch baking dish.
In a saucepan, stir together the Alfredo sauce and chicken broth. Season with basil, marjoram, garlic powder and thyme. Heat until warm over medium heat. Pour over the shells in the baking dish.
Bake uncovered for 30 minutes in the preheated oven. Let stand for about 5 minutes before serving.
