My Bigfoot is a 1988, and had no A/C from the factory. As stated, the A/C are designed to fit the 14-1/2"x14-1/2" standard roof vent. It is very common to have an A/C installed after the TC is purchased. I installed my A/C (about 2005), and the hardest part was getting it on the roof of the TC. At the time, Camping World wanted $100 for the install.
If the TC is pre-wired, which it probably is, look in the overhead cabinets on the side of the TC, there is a very good chance there is an electrical junction box. The box is probably on the side of the TC the converter/charger is on. You will see the Romex going in, but nothing coming out. If you have a ceiling vent, it will be inline with it. On my Bigfoot, they left a piece of foam insulation out just big enough for the Romex to go from the junction box to the vent opening.
It is simply a matter of removing the existing vent, fish the wire through to the junction box, install the A/C over the existing hole and complete the wiring.
My TC did not have an existing vent, so I had to cut the hole. I first located the junction box, then went across to the center of the roof. With a stud finder, I located the 14-1/2"x14-1/2" roof vent framing. The vent was framed with 2"x2"s, but the hole was not cut. Unfortunately, I have no photos.
Wayne