I am afraid that I can't give a recommendation based on my personal experience. I find two campgrounds with very high ratings, but I have never stayed in either of them. There is the Horseshoe RV Park near Buffalo, Texas; and Hidden Valley Oaks RV Park near Rockdale. Each of these gets a 9 or excellent rating on RV Park Reviews.
If you "swung" your route a bit north there is Tyler State Park just off I-20 which is a great place to stay with full hookups. You would almost surely have to have reservations to get in there as it is quite popular. Texas does have a new reservation system which allows (in fact, requires) site selection.
Around Bastop, Texas which is a bit west of US 77 there are two state parks, Bastrop State Park and Buecher State park which actually back up to each other. There is also a very nice campground on Lake Bastrop operated by the Colorado River Authority. We have stayed in every one of these.
I wish I could be of better help, but this is the best I can do.
I will add that on our way to the valley we usually swing a bit west to stop at the Choke Canyon State Park, just west of Three Rivers, TX. There is also a very nice state park at Goliad--very rich in Texas history--southwest of Victoria. We've stayed there a couple of times.