Mark the opening hole on the top of the roof with the marker. Use the template included with the air conditioner to mark the size of the opening.
If your air conditioner does not come with a template, read the manual for the size of the opening.
Drill a hole with a metal drill bit on each of the corners of the marked opening on the top of the roof.
Cut open the hole in the roof with the reciprocating saw. Insert the saw blade through the holes made with your drill, and cut along the marked lines.
Place the top part of the air conditioner on the opening. Secure it in place using the included fasteners, using the electric drill with driver bit or a screw gun.
Read the manual for specific installation instructions, because they vary by vendor.
Seal the trim of the opening with silicone sealant. Place a thin bead of silicone around the opening and the air conditioner to make a waterproof seal.
Install the interior panel of the air conditioner inside the trailer. Read the manual for directions, because installation methods vary widely by manufacturer.
Plug in the air conditioner in a trailer outlet. Turn on the unit and test it.