sewage slope needed

I am in a 80 foot mobile home with a bathroom at both ends of the house. The drainage is clogged between the front half and back half of the house. Should there be a downward slope to where the T is that goes across the bottom of the house then toward the drain where it goes into the city sewage system. How much of a slope is needed for the drain to flow properly.
Donna

1/4" of per foot of pitch is a standard. 4" lines are allowed 1/8" per foot in some areas. A clog can have as much to do with the type of fittings used and how the connections are made.