Those older coaches were really not designed to have AC in them, because it was a pretty new thing at the time. Later on it did become an option, but they did at least prep it with extra support.
If you really want to give it good support, you could remove the AC and install some more bracing inside the hole.
Another (expensive) option would be to replace the AC unit with a newer, more lightweight/efficient model. If you use the AC often at your house where you pay for electricity, this may be a cost-efficient move.
Also, the 15 1/2' SB does have the advantage of having a lot of windows, so, as Norm & Ginny have found, AC really isn't necessary. I'm sure you could find a good sized roof vent (more like an escape hatch) to fit in the hole. An exhaust fan might work well too to draw air through.
Jon