We have the same version as your camper. Ours is an '82 and it came without an air conditioner as well.
Fortunately the trailer was designed for the installation of an air conditioner. If you take the cover off the central air vent from the inside you will find AC wiring for an airconditioner. I assume the other end of this cable runs to the AC box or at least near it.
The breaker box will require the addition of another breaker for the air conditioner and probably, if your's is like mine, the installation of a 30 amp power cord. These breakers can be bought on EBay. If you pull out the breaker and enter its name into the search on Ebay you'll come up with several suppliers. I recall about $8
I suspect that an air conditioner will mount in place of the overhead vent.
In our case we have never found the need for air. Probably because we live in NH and also because we travel from September to May. Even in the 80 degree weather of Florida this April we found simply opening the windows kept us cool enough. I lov3e the windows because they virtually open all the way unlike most RV windows.
We also carry a very silent small fan that we bought at the Wal. In addition we mounted two muffin fans in the bathroom and kitchen vents.
(Actually we used the air conditioner wiring to power the muffin fans in the kitchen area.)
Norm and Ginny Milliard
1982 Sunline 15.5 SB
2004 Honda CRV 4 cyl, manual