There are a lot of Texas Boomers about, so I know you will get more suggestions. I don't know if San Marcos is really "Hill Country"
. Our favorite park in the Hill country is down the road a piece in Kerrville. Its Buckhorn RV PARK, link to it
Here. Right off I-10 in Kerrville. Exceptionally clean well manicured RV Resort. Close to all sorts of Hill Country attractions including Fredricksburg (Yum German Food) and Bourne.