Unfortunately the Bay Breeze is installed very differently than a modern unit. Instead of a 14 x 14 opening, it is installed with just two small holes through the skin. One is for the motor shaft and the other is for the refridgerant lines.
There is a lower unit and an upper unit. The lower unit is attached to the interior skins with a couple dozen pop rivets. It contains coils, an interior fan blade and the control unit/switch and electronics. The upper unit is attached to the exterior skin with about a dozen screws (each of which leave a hole that will need to be plugged with a rivet) and contains the compressor, the fan motor and coils including the refidgerant fill valves (I'm not an AC guy so I don't know the technical terms for all this).
If you are going to install a modern unit in its place, you will need to cut a 14 x 14 opening and add some material to support the weight. The Armstrong units were designed to distribute weight accross the skin of the trailer and were contoured to fit the curve. Modern units require support between the interior and exterior skins or it will collapse the roof of your trailer.
Here are a couple pictures of the interior removal. I forgot to take photos of the exterior. That thing weighs a ton, so be careful if you take it down. Since we made the decision to scrap mine, me and a buddy just heaved it over the side. You will need to get an HVAC guy to recover the Freon (if you don't want to violate Federal laws) then you can unscrew (or cut) the lines to take the two units apart.