Some musings about why they took the vehicle & trailer into Mexico.
Either they were planning to head to somewhere they know to hide out in or they were planning to sell both truck and trailer.
First of all, it would be so unsafe to leave that trailer unattended. Just as the Butler's were killed, the perps, too, would become theft targets so they really have to head for somewhere they know they can be safe: Maybe in a gringo ex-pat community. Maybe to some secluded hideout. But they have chosen a border crossing that isn't really good for anything but heading directly south. Not west toward the mountainous regions of Chihuahua where the killings of the young families occurred. But they still have a very very long drive to the better known ex-pat hideouts.
Secondly: It makes far more sense to sell the rig. That vehicle will stand out like a flashing diamond. You would not want to be traveling in Mexico as a tourist with it. Someone bad will want it.
Anyone experienced in Mexican travel knows better. It's better to sell it and sell the trailer. Take the money, buy some cheaper, more beat-up truck. If they think they can get farther south out of Mexico through Central America - good luck to that!. Border crossings are difficult and subject to bribes when your paperwork doesn't quite add up.
I think they were looking to sell, hang out for a bit until the trail cools and then re-enter the US in a bit. Try some fake passports. Good luck with that, ha ha
Or they are just really stupid and think they can drive that rig through Mexico as if they were tourists and not attract the wrong kind of attention. I can't wait to find out who they claimed to be on their passports.
Whatever it is, I want them captured alive and extradited to the US for LWOP. The Butler's family and friends all over the world need to know and confront them.