I spent a few years in Texas and often travel by road cross country. I do have to say, finding a dollar store chain of local is a blessing in the big empty in-between.
Healthy food is easy to find at these stores. I can put together a perfectly good home cooked meal from Dollar General no problem. Baking goods, dry goods, dairy, produce... again the selection is smaller than a larger grocer, but it's all there.
I don't live anywhere where they have either a Dollar store or a 7-eleven, I've only learned about the 7-eleven concept on my last trip to Bangkok.
Dollar Tree (and Family Dollar IIRC) are a bit different because unlike Dollar General each item is actually a dollar. They're a bit more akin to Daiso, if you visited any of those in Thailand, though again with a bit more food (though less than Dollar General, and in smaller packages).
That is surely because a 7-11 in say Japan, Korea, or China are part of vertical, dense infrastructure. You don't have miles of space in between things.
And I would be curious if Dollar General has that much more of a selection compared to some of the big 7-11 I've visited in Seoul.