Someone alerted authorities about the trailer in 2022 after seeing small fingers reaching through gaps in the doors.
Police stopped Samuel Bateman's vehicle as he was driving through Flagstaff and found three girls inside, who were aged 11 to 14 at the time. The trailer was enclosed with a makeshift toilet, a sofa and camping chairs.
In the federal case, Bateman was convicted of coercing girls as young as nine to submit to sex acts with him and other young adults, and for scheming to kidnap girls from protective custody, the story of which is the focus of a Netflix series, Trust Me: The False Prophet.
Bateman previously claimed to have more than 20 "spiritual wives," including 10 girls under the age of 18.
He testified in his own defence in the case, telling jurors he would never harm the people he loves.
Bateman acknowledged during cross-examination that he knew the girls were in a hot trailer for hours and the ventilation wasn't good.
"I just trusted myself as a driver," he said. "I ask God to bless me every time we hopped in that vehicle."
Jurors in the state case weren't supposed to hear about Bateman's conviction in federal court. The judge barred the evidence from being introduced. But Bateman brought it up several times as he represented himself, leading the judge to strike the comments from the record.
The jury delivered the verdict on Friday, convicting him on all three counts of child abuse.
Authorities said Bateman, a self-proclaimed prophet, travelled extensively between Arizona, Utah, Colorado and Nebraska as he built an offshoot network of the Fundamentalist Church of Jesus Christ of Latter-day Saints.
He and his followers practiced polygamy, a legacy of the early teachings of the mainstream Church of Jesus Christ of Latter-day Saints, which abandoned the practice in 1890 and now strictly prohibits it.
Bateman was one of the trusted followers of Warren Jeffs, who previously led the sect and is serving a life sentence in Texas for sexual assault of children.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
Lifeline 13 11 14
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
