I have a mobile home with the typical roof vents for sewage. After setup, the system is not draing properly. Everyone says I need another vent for sewage in addition to the one through the roof. Can I install one directly in the main sewage line, specifically near the clean out? If so, what exactly will I need?

Your home came out of the factory with all the vents you need. If you have draining problems adding more vents is not the answer. Your existing vent or drain is becoming blocked and needs rodding. Since it sounds like your home may have ben recently moved and set up I'd suspect that whoever did your set up may have made a mistake. It is also possible that you have been connected to a bad sewer drain in the ground. Does anyone know that the drain worked fine before your set up?