Usually it is plenty warm, very little rain that time of year. You will want nothing but shorts and tees to wear and find plenty of cool places to set a spell. Unlike AZ, with 10-15% humidity,,, SA will be in the 50-70% and Corpus Christi and where I live, George West in the 70-90% range. So it will be hot. But you will be able to have a great time. Bring sun screen, sun glasses and camera.
The Alamo Fiesta RV park is West of San Antonio on the edge of Boerne, and very easy on and off I 10. (We stay there when we take RV for some reason, daughter lives in Boerne.) You almost never know your out of SA. But it is very easy to get to downtown...30 min., or you can park truck at mall on outer edge of SA and catch bus downtown. I think they have a $3 per person all day pass. Works really good... can tour all the downtown area, see all the sites, and almost never walk more than 2 - 3 blocks.
Keep us in mind.
Jim M