Gas and Oil Home Heating Furnaces - Air Duct Problems

I recently went under my New mobile home and found one of my 10" round air ducts laying on the ground cooling the underside of my home. I noticed that my home has two permanent rectangle vents that run the length of each side of my home. And that this vent was connecting the two permenant vents together. I also noticed that the outtake air duct from my AC Unit was going to this same duct. I'm not sure any of this was put in properly and was wondering is their anywhere I can look to learn how to properly hook up the duct work for maximum air flow to all rooms in my home. Also is the intake and outtake ducts going to my unit supposed to be the same size. Thank you very much, my air flow is terrible and I have a 4ton unit
Jackie