I would describe the Texas hill country as a roughly 50 mile wide strip ranging from Uvalde on the south end, up through Kerrville, and Frederickburg to Waco at the north end.
Off hand I can't think of any major waterfalls in the hill country though there are multiple spring fed rivers that are popular for tubing float trips. These include the Rio Frio at the south end and the Guadalupe in the central region. Garner State park is on the Rio Frio and is one of the most popular state parks in Texas, I have read RV check in can take over 2 hours during the busy season, there are also numerous commercial RV parks along the Rio Frio (I counted over 25 in a 12 mile stretch of road just north of Garner state park a while back). Also in this region is Lost Maples State natural area. If you are considering Fredericksburg, think about staying at Lady Bird Johnson public park, it is a 300+ acre public park with golf course nature trail,etc. When I was there in May they were in the process of renovating the RV park, installing new power posts, and building a dog park next door. The local general aviation airport adjoins the rv park, but don't let that worry you as there are few if any night time flights, and even few daytime flights. If you have any interest in World War II history, you must go to the National War in the Pacific Museum in Fredericksburg, plan 2 full days to see the museum, there is a reason the tickets include 2 days worth of admissions.
On the nature side of things check out Waco Mammoth national historic site for the fossil pit https://www.nps.gov/waco/index.htm
If fossils interest you may also check out Dinosuar Valley state park at the north end of the hill country in Glen Rose https://tpwd.texas.gov/state-parks/dinosaur-valley