I'm looking for the plumbing schematic for a 1986 Fleetwood single wide mobile home, 2 bedroom, 2 bath. We purchased a used home and need to locate sewer vent, as the house has a second roof over it and we are unable to locate when to cut a hole so it can vent properly.

I would not trust a schematic if I were going to cut a hole in the roof. Aren't there gable vents in the new roof structure that you could gain access or at least a visual confirmation of the existing vents ?

Go under the trailer and look. There will be at least one line going up that doesn't have a fixture attached.

The pipe you are looking for will most likely be a large black pipe 21/2" to 4" in diameter near the bath rooms.

You are joking, right? Mobile homes do not have "schematics".

Mobile homes do not have ANY pipe larger than 2" above the floor, and any vent is usually 1 1/2".

Look inside of closets etc for a pipe running to the roof. There may be more than one depending on whether it is a mobile home, park trailer, or travel trailer. Trailers will holding tanks will usually have the black and gray water vented separately like mine does.
YES, there would have been a plumbing schematic, along with electrical plans, etc. Unfortunately Fleetwood went under in 2009 and all of the documents pertaining to the trailer business have been dumped.

My motor coach DID NOT come with a "plumbing plan".

No, it's not provided as normal course to the consumer, but they have them at the factory, and if you call customer service they are usually happy to send them to you. That's IF the company is still in business.

Re-Modeling homes for 35 years.
I am NOT a licensed plumber.

Or, you could call a plumber and he could draw the schematic after a few minutes looking at the installation.