The only place we have stayed around that area is Yogi Bear Jellystone park in Canyon lake. It's a little pricey but a nice place. My MH is 15 years old and they did not ask of say anything about it. There are also some state parks and COE parks in Canyon lake. It's a little west of San Marcos. The only problem is that you probably can't travel as fast on the roads as you could on I35 so that area may not work for you.
If you have a smart phone you may try to down load the Allstays app. You can zoom in on San Marcos and see all the camp grounds. Touch the location and it will give you all the info about the park,phone #, web site, reviews, even google maps with direction as to how to get there. I use the filter to reduce the clutter on the map. Good Luck