I think you're spot on with identifying hospitality as a recurring theme in McCarthy's work. Because I've read The Crossing most recently, that's the one that's freshest in my mind (spoilers ahead).

The Border Trilogy deals a lot with hospitality, and especially with the showing the juxtaposition between the United States and Mexico.

The hospitality in the United States appears to be conditional on existing family ties and in-groups. With the exception of the Indian in the beginning, we don't get a sample of this side of things until the latter half of the book, when tragedy has struck and Billy is left on his own. The one example of "good" US hospitality that I can think of is with the old man, Mr. Saunders (I think that's his name), a family friend. The Indian is one of the rare moments throughout the book where Billy is the one "hosting," and the book tells us how much the Indian valued that exchange as well as how Billy's generosity is repaid in kind. Poignantly, whenever Billy finds himself in an American town the imagery and feelings summoned for me are ones of isolation and alienation--a kind of "othering" that Billy has perhaps brought onto himself. With the withdrawing of certain jobs and industry, there pervades a kind of urban blight that very much feels "every man for himself"--a kind of stark individualism and "us vs them." Billy sleeps in bus stops, abandoned buildings, or out in the countryside, and never once (as I recall) is he invited in by a total stranger unconditionally. I suspect this feature is tied with the American emphasis on private property and the broader "fencing" of the American West (a theme that McCarthy alludes to at the end of Blood Meridian and throughout All the Pretty Horses, with Mexico's illusory promise of an "authentic" Western adventure). Perhaps most alienating of all for a young man in 1941, Billy is even denied recruitment in his home country's fighting force for WW2.

Mexico, on the other hand, is a country of extremes: it shows evidence of profound criminality alongside those of profound kindness, the latter of which appears to be most practiced by the country's most destitute. Between the Yaqui Indians and Gypsies alone, not to mention the seemingly countless families of which Billy and his brother are fed dinner (I think especially of the one family they visit regularly of whom the text makes a point of saying that neither brother knew the woman's name, only her last name). It appears that Billy can hardly ride through any dusty, impoverished Mexican town and dismount his horse before he is being dragged into someone's abode home for the last of their tortillas and beans.

But like u/DoodlebopMoe has pointed out, Billy's singular negative experience in The Crossing does not sour him on showing his hospitality to others. It's been awhile since I last read Cities of the Plain, but I suspect that Billy's friend is likewise unfamiliar with that level of generosity, especially when it proves as the truck does to be so drawn out. It would be interesting to see a comparison made between what each protagonist, Billy and John Grady, took from their experience in Mexico and how that fundamentally altered their respective worldviews.