Im helping a buddy tie his drainage to the sewer line but when I got under his trailer I found a fitting that I have never seen before it is 4" pipe and then on the end of it has a 7" OD fitting with some sort of rubber lining... any ideas

Not really... trailer parks can come up with all kinds of ways to connect to existing drainage pipes.

I would ignore that transition fitting and then cut a new WYE fitting into the trailer's drain line. You can use SHIELDED MISSION STYLE CLAMPS (see image) to cut in and reconnect all the piping/wye fitting.

More questions? Just let us know, OK?

Mark