Of course you will have to follow whatever directions that come with your AC.
Adding a second AC to garage area.
I have a 15k BTU but needed a little more for those really hot days. I have been wanting to do this for a while but just did not want to spend the big bucks for a new AC. I have been watching Craigslist and finally found a unit the same brand and style. A Dometic Duo Therm that was in really good shape. It is a 11000 BTU with heat strip. The Price was right so I bought it.
My Toy Hauler has 50 amp service and was prewired from the factory for a second AC which included a seperate circuit breaker and electrical junction box at the rear garage vent.
First You have to remove the vent and all the sealer that the factory poors over the screws on the roof.
Don't get anything sharp near your roof. You can use a metal putty knife to remove the sealer as long as you stay on the top of the vent flange...Repeat don't go near the ceiling with this tool.
When you expose the screw heads you can use your cordless drill to get them out. Mine were square head and there were alot of them.
Once you have the screws removed you can use a plastic putty knife but even these I would take off the corners.
When you think you got as much sealer off as you can use the plastic putty knife to pry under the vent flange.
When you get the vent removed you will have to clean up the area around the vent hole.
You may want to seal the holes left by the old vent. These should be coverd by the AC gasket but you can't be too safe. I have an aluminum roof. I used a strip of aluminum foil tape. It will be under the AC gasket so should be OK. I have heard that a little sealent on each hole would be ok or even putting flat head screws back in the holes till they were flush. I also read that if the gasket sits over the holes you dont have to do anything. I did not like that idea.
Prepare the AC. I cleaned it all up and cleaned the shroud.
I wish I had got a picture of us lifting the AC up to the roof. Took four guys, two 8 foot ladders. So you don't damage the AC gasket you dont want to slide the AC along the roof. Get it over the hole as close as you can then you can center it from below by lifting up and moving it a fraction one way or the other.
Work on the inside.
I have a 6 inch ceiling so I had to order the optional bolt and duct kit. There are four bolts that clamp the AC to the Roof. Mine were 1/4"x 8" . The scary part is just how tight should the bolts be? The instructions say 50 inch lbs of torque or till gasket is 1/2". I did 35 inch pounds. It passed the water hose test. I ran wire from the factory provided junction box and used aluminum duct tape on all cracks and voids. I installed the inside control panel.
Check circuit breaker