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 minutes, until al dente. Drain and place on a greased baking sheet to cool.
In a medium bowl, stir together the ricotta cheese, crab, spinach and onion. Spoon into a large zip-lock bag. Cut off one corner of the bag and squeeze the stuffing into the manicotti shells. Place in a greased 9x13 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 until slightly browned. Let stand for about 5 minutes before serving.