I travel across the border to the US all the time with my trailer and have for many years. My advice is - do not bring firewood, fresh fruits or vegetables or meat. Everything else should not be an issue and they rarely ask me to go for a secondary inspection or even look in the trailer. Firewood is becoming a big issue due to bugs. I used to bring it all the time but not anymore. It's not the Customs guys and gals but USDA (US Department of Agriculture) that will likely be looking for fruits, vegetable and meat. We have had citrus fruit taken in the past, tomatoes, etc.
If the fruit is labelled or packaged with the country of origin it might be ok, but over time I have found it's not worth the effort. Buy it when you get there.
An interesting note is that while they may confiscate items like whole tomatoes, they are usually ok if they are cut up like in a salad. I guess you would not leave bugs in your salad.
As others have said, be honest, brief and answer questions without a lot of fanfare.
I always take off my sunglasses, put the left rear (tinted) window down as I approach so the officer can see in the back seat, and have my passports open and ready to hand to him/her.
Hope this helps.
-2013 F150 4x2 SuperCab 3.5EB
-2011 Jay Flight 22FB