Cook manicotti shells according to package directions; drain and set aside.
Saute zucchini in butter in a large skillet 6 to 8 minutes. Remove from heat; add cottage cheese and 1 cup cheddar cheese, mixing well. Stuff manicotti shells with cheese mixture; arrange in a lightly greased 12 x 8 x 2 inch baking dish.
Combine tomato paste, water, and sloppy Joe mix, stirring well; pour over manicotti. Cover with foil, and bake at 350 degrees for 30 minutes.
Remove foil, sprinkle with remaining cheddar cheese, and bake an additional 10 minutes.