Building a covered porch onto a mobile home would be basically like building a covered porch on a regular home.
To build the roof on the mobile home you need to get some rafter ties that are made of metal that you can get at home depot or other hardware stores and then screw those ties into the studs of the mobile home.
Make sure you find the studs before screwing the rafter ties into the mobile home and be sure there's no electrical wires in the area where you're mounting the rafter ties.
That's how you make the connection of the mobile home porch roof to the mobile home. You use rafter ties to make the roof connection to the mobile home.
Then just build the bottom portion of the porch the same as you would build the porch when building it onto a regular home.
Do use flashing between the mobile home roof and the covered porch roof to avoid any leaks coming in and running down into your covered porch and into your home.