When crossing state borders so they check uhauls for sex trafficking?

I know they stopped my brother when he was driving a u-haul truck into Florida to make sure he wasn't taking in any plants. In fact, they confiscated the house plant he had with his furniture. They're very concerned about pests and diseases that could destroy the crops. It sounds like California has some of the same laws. I don't know where the story about sex trafficking came from. If the people who said they were stopped and the police told them that are reliable, then I guess it's true. But it sounds made up to me.

First, you mean state lines. Borders are between countries, not states.

I haven't crossed -every- state line, but there usually is nothing at a state line except a sign: "You Are Now Entering [State]"

Here in California we have stations at state line crossings where they stop cars to ask if you're carrying any fruits or veggies. And even at those, the last few years they've never stopped me, just waved me through.

Never happens

Why are you worried? If you just have possessions in the trailer....