Google Maps puts the shortest almost directly north from San Antonio - 1203 miles - via I-10 to Junction, TX then 83/153 to Sweetwater then 84 to Lubbock, TX and I-27 to Amarillo then 87/287/385.
I didn't calculate the I-25 route but it looks way out of your way.
For 16 years of full-timing we traveled secondary roads and rarely interstates and never had a problem.
You're going through a lot of farm country and many drive diesel trucks or need diesel for their equipment. Every town will have diesel. It's not rare. Yes, you 'could' have mechanical issues but think positive.
Make sure your tires are in good condition and I'm sure you have roadside insurance. Really... people rarely have mechanical issues if they take care of their equipment.
As TonyDi said above, go with what you'll feel more comfortable doing. Have a good trip!