First of all, check out the UHaulCamper group on Facebook. Most of the UHaul
owners are now using that site almost exclusively for maintenance and repair questions. You can request membership and get a pretty quick response from the group moderators (my wife is one!).
Regarding the CT13, I don't believe that any of them came with the swamp cooler. That was typically found on the larger unit, the 16' VT. If it was there, it may have been added later. In any case, it is very doubtful that any added support was put in place.
The smallest ACs are really too heavy for the roof of the CT without additional support. The support might be a pole from the inside kitchen counter to the roof or some type of weight
distribution bracing across the inner ceiling. Otherwise, you run a big risk of having the roof sag. You may also need to put in an additional circuit and breaker for the electrical
Other CT owners have put AC units through the back window and under the front seat. The front seat option is more aesthetically pleasing but either would work. The rear window option will require some support from the rear bumper to the AC unit to help hold it up. Some owners put it in the window permanently and others install and remove the unit with every trip. You can find lots of help with this issue on the UHaulCamper page!
Best of luck! If you can get this great little camper for a reasonable price, you can work out the AC problem. Have fun UHaulin'!