kreativekvs209, how are you going to construct the wall where the back door will go? Could you explain your reference to ‘metal rods’ ? What kind and size of door will you get? I might go with a steel plain slab ext door in a 2×4 wall.
It looks like you are framing out a 2×4 wall over the top of the existing aluminum one which will reduce overall width. Why not just go with plywood onto of the original aluminum studs?
Also, do you have a rear bumper?