How old is your son? And what kind of activities do the two of you like?
Have been to the hill country around Fredericksburg every year for many many years. Lots of scenic areas there and all over the hill country (Marble Falls, Burnet, Llano, Mason, Kerrville, Bandera, etc). And although we always pass thru Johnson City on the way, we have never stopped there.
However, we really like the
Kerrville Schreiner Park in Kerrville as our RV destination. A very large park (formerly a state park) with a relatively small number of RV sites, so it is not very crowded and sites are not side by side. And deer often roam thru your RV site. But that is quite a ways from your destination.