None of the people found Friday required hospitalization, Victoria County sheriff's officials said.
"At this point, we're just providing humanitarian aid," said Carol Bludua, the office chief of staff. "We giving them water and whatever they may need."
The driver of the truck is in custody, Bludua said. The rest stop is about 10 miles east of Victoria, or about 120 miles southwest of Houston.
Earlier this week, a trailer was found at a truck stop a few miles south of Victoria with 17 bodies inside. Another immigrant died at a hospital later, making it one of the deadliest smuggling attempts in U.S. history.
The driver of that truck, Tyrone Williams of Schenectady, N.Y., told authorities he struck a $2,500 deal with two men Tuesday to haul 16 immigrants from the border town of Harlingen north into Texas. Instead, authorities say, more than 100 men, women and children from Mexico and Central America were loaded into the virtually airless truck.
Williams, 32, told a federal customs agent that he didn't know how bad the conditions were in the trailer until he pulled over early Wednesday. He said he unhooked the trailer and fled to Houston, where he was arrested.
As of early Friday, he did not yet have an attorney.
Williams at first claimed he was hauling an empty trailer but later changed his story and admitted taking the money from two smugglers he identified as Joe and Abel. He said they paid him and loaded the immigrants aboard his trailer themselves. He then drove off with a woman named Fatima he said he met in Harlingen.
Federal authorities said they are still looking for Joe, Abel and Fatima, who vanished from the Houston hospital.
Williams told authorities he picked up the immigrants late Tuesday and began driving. Later one of the men called and told him they would pay an additional $2,500 if he delivered the immigrants to Houston.
Williams pulled over his rig as he approached Victoria because he saw a light dangling from his trailer, the customs agent said. When he pulled over "he heard banging and screaming coming from the trailer," the agent said.
"He heard a female voice screaming over and over, 'El nino!"' according to the affidavit.
Authorities say Williams and the woman went into a gas station and bought 20 bottles of water for the immigrants. The agent said Williams told authorities he saw the dead immigrants when he opened the trailer's doors to give them water.
Williams says he left the doors open, unhitched the trailer and left. But authorities say sheriff's deputies had to open the doors themselves.
Eduardo Ibarrola, Mexico's consul general in Houston, said survivors told him they felt the lack of air "almost as soon as the trailer started" and that after 2½ hours, people inside were already dropping dead.
"They believe that if they had been locked inside for just a few more minutes, everyone might have died," Ibarrola said.
Most of the immigrants are from Mexico, but some are from El Salvador, Guatemala and Honduras, the consulate said.
Forty-five survivors remain at a makeshift shelter in Victoria, while six others remain hospitalized, including two in intensive care.