For Lake Buchanan, check out
Canyon of the Eagles. While much of what they advertise is a resort-style setting, they have a nice loop with a number of slots for RVs. The price is a bit steep, but a very nice setting.
This was the first place my wife and I stayed after purchasing the trailer in August of 2012. We've been wanting to go back, just too many choices in parks in Central Texas!
Another option is the next lake to the south on the Highland Lakes Chain, Inks Lake. There is a state park, probably one of our favorites, aptly named ...
Inks Lake State Park.
Good luck with your search,keep us posted on where you end up staying.