I have an old mobile home and need to add AC. I have a gas heating furnace and that is how I heat in the winter and want to keep it but, want to add Central AC to the same furnace. Can the two be on the same furnace, and can anyone tell me how to do this.

Yes I think you could if you just ran the air conditioner duct into the heating duct that already exists with a valve inside (That you would turn so the A/C couldn't flow into the heater) Then you would just switch the valve depending on the temperature.

Just install a coil into the supply side of the duct usually right under the furnace IF your furnace is a down flow model like most mobile homes are.