I have asthma and don't do well in heat. I'm OK as long as the nights cool down, but if the nights stay hot and still, I'm in trouble. So, I go to the mountains in the summer. Don't get the over 100 degree days, or the humidity, and the nights get a good breeze. I do things in the morning and early evening and siesta during the hottest part of the day - put my chair in a nice shady spot and read for a couple hours.
I am camping though not sightseeing. If sightseeing, you might try to keep to indoor sites midday to late afternoon - things like museums. Do outdoor sites in the morning before the heat takes over. Pick RV Parks a bit further out from the "bad air" center and spend the evenings resting. Not trying to do as much as you used to in a day will help. Separate outdoor activities with indoor rests (eat more small meals throughout the day so you can sit inside air conditioned restaurants).
Stay hydrated and check your meds for sun exposure issues.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)