Keep in mind that the only requirement for a border agent is to have a GED equivalent. There is no set criteria for questions that agents ask.
Since we live in Mexico and cross to the U.S. on occasion we have more than 30 years experience. In our party there is a U.S. citizen with permanent Mexican residency and a Mexican who has worked for the U.S. government, been a Fulbright four times over, worked as a French interpreter for Mexican and U.S. dance groups in Europe four times.
Each crossing requires we take six months each of salary stubs, electric bills, phone bills, copies of the properties we own in the U.S. and Mexico. Sometimes that isn't enough and they send us back.
To show how the U.S. border agents are very similar to TSA, in less than 15 minutes we are in line for another border bridge. The agent asks no questions, and says, "go ahead and have a nice day". Or, we wait for shift changes which works too.
What we have found is that is an anglo encounters a Hispanic agent there is no issue, the same for the Mexican encountering an Anglo agent. Reversed, that's when we have issues.
Forget the rv, when we take it we can sit at the border crossing for over two hours. Questions like, Why do you live in Mexico? Why aren't you carrying cash? Last time you crossed in a smaller car, Did you just buy this one?
Forget the check points north of the border that by law, should only be 21 miles north. Some go as far as 100 miles into the U.S. Why is that an issue? Because border agents are supposed to be working the border? Why didn't they catch the illegals at the border? The whole thing is a mess.
Soon, all Americans will be required to carry a U.S. passport in the U.S. If memory serves me right, they did this in 1938 in a certain country whose leader turned the world upside down.
Sorry OP, this was supposed to be your rant :)