The only stretch I really ever had issues with finding parks is from Victoria to Corpus Christi. Victoria was never a problem until the oil boom, Corpus Christi is simply not my cup of tea YMMV.
From Waco If you want to keep to the key snowbird route(s) you can either head to San Antonio or Rockport. Since this is your first time head over to Rockport on 77. Your driving nerves will thank you, and you can always do San Antonio on your way back (TAKE THE NEW TOLL ROAD NORTH from there to past Austin.)
If you get hitch itch in Rockport (we did) head down to the RGV on 77. If you're young (body and/or mind) and like a party town feeling head to SPI, if you like bingo, happy hour, and dulcimer lessons head west on 83 (lot's of park models.) If you just want a good taste about the valley hold up somewhere in Harlingen to San Benito and take the TV around.
This really seems to be a slow year for snowbirds, don't think you'll have any issues finding parks along your route. Our park which is 100% full by now has many spots open.
BTW, you're missing Hill Country which many enjoy. Best time for that area is October/November & March/April.
