Working folks have different driving numbers than retired. I've driven ten hour days towing and I'm fine but I don't like to do that more than two in a row before I take a day of four hour towing with sight seeing or a full stop.
You should not expect to get to Tucson from San Antonio in a day. The 18 hour trip will take at least 20 hours towing and probably more. There isn't much between El Paso and Tucson but I would look for a place to stop half way.
What is your tow vehicle? The climb is real going in to San Diego and often windy as well. That will make a difference. And gas up in Yuma before gas jumps 40 cents per gallon.