Jody Kuchler, who had been driving behind the pick-up 75 miles west of San Antonio, also said 20 year old local man Jack Dillon Young had apologised following the incident.
Mr Kuchler said: "He said, 'I'm sorry, I'm sorry. I was texting'. I said, 'Son, do you know what you just did?' He said, 'I'm sorry. I'm sorry'."
Mr Kuchler also claimed that he had called police after seeing the truck moving erratically prior to the crash, when he warned officers, "they needed to get him off the road before he hit somebody".
Officials have said Mr Young, who remains in hospital, appeared to have crossed the centre line of the road.
The National Transportation Safety Board (NTSB) confirmed distracted driving will be among the avenues of investigation.
Members of the First Baptist Church comfort each other after hearing about the tragedy
Texas is one of only four US states which do not have a state-wide ban on texting while driving, however, such legislation is currently on its way to the upper house of the Texas State Legislature.
The victims of the crash near the town of Concan were all members of the First Baptist Church in New Braunfels, Texas and had been returning from a three-day retreat.
In a statement posted after the crash, the church said: "It is with heavy hearts that we confirm that thirteen of the fourteen passengers in today's bus accident were called home to Jesus."
According to the NTSB, most - if not all - of the occupants of the van were wearing seatbelts.
They were all aged above 60.