We have gone 81 to Austin for the last 3 winters. I echo the recommendation to go 20 rather than 40 West. 40 has some serious climbs!
If it's cold and we haven't un-winterized, we use the Microtel in Christiansburg for about the same price as a KOA.
We like the Allstays apps and use their multiple versions. They are as good as the crowdsourced data that is given to them -- better in some areas than others.
You have heard of Pecan Grove in Austin, right?