Some things I would wonder is: Are you getting bad reviews because of no A/C? If so, those might hurt your appeal to an online shopper and could hurt your ability to consistently fill the unit(s). I would look at the potential opportunity cost lost due to vacancy as a result of not having A/C. For only a few hot days out of the year it may not be a big deal though. I agree with @Luc Boiron my wife and I usually avoid places that show no A/C.
Is it possible to just get window A/C units in each bedroom? That can get a little pricey of course. However, another less expensive alternative, in terms of initial cost and electric usage, might be portable swamp coolers/evaporative coolers. They might not get as cold as A/C, but can certainly lower the temps as much as 20 to 30 degrees, usually portable, and may avoid complaints without a huge investment. Check them out here.
I think whatever you do I would just keep in mind that you want to make your customer/tenant happy and make sure they have the best experience as possible. This reflects well on you and they are more likely to tell their family/friends/acquaintances to stay in your place(s) as well.
Best of luck!
