To vent or not to vent?
I am a licensed master plumber.
I believe I answered your question in another forum you posted in. I can not copy the link or copy and paste the post, so I'l try to re write it
You can not run 18 feet with the pipe horizontal because you will kill the vent, every trap and trapped fixture needs a vent which can be achieved in any number of ways,
1) Air admittance valve
2) Wet venting
3) Direct vent
First lets say we will use a 3" line horizontally (Sufficiant for toilet and sink, 4" is overkill) Now try to run the line from the toilet to where the sink line will drop down under the floor. Here we should use a 3" x 2" tee wey so we can connect the sink drain in with the toilet. Make sure the 2" side is at the top or at minimal 45 degrees off center (It can not be laid flat as this will kill the vent). Now we can tie in a line for the sink. Run your pipe up through the wall and use a 2"x1-1/2" x 1-1/2" sanitary tee (The 2" opening is at the bottom, the 1-1/2" will point upward to the attic, the other 1-1/2" will catch the sinks drain) Next run 1-1/2" pipe up into the attic, turn a 90 (Elbow) toward the existing vent that goe's through the roof and tie it in (At this point you will want this sanitary tee to be installed upside down for venting) Back under the floor finish running the horizontal and tie into the existing drain. This should work fine, if not you can email me and tell me why it will not work and I can guid you from there. Good luck! And bear in mind that any line used as a vent can not be run horizontally untill it is 6" or higher than the flood rim of the highest fixture it serves