I am a US citizen and travel to Canada about every 4 months. Never with a trailer though. Never have I had an issue with Canadian customs. They are always professional and do their job.
Coming back into the US has been more of an issue. Not a big deal, but kind of funny. One time the customs agent asked where I lived. I told him MA. He then added that before that you lived in VT. I said no. He said that's what your passport says. Okay, 66 years ago, I was born in VT and lived there for a year. That ended that.
The most serious issue happened when the agent typed in the wrong info about my
license plate. That led to being pulled aside for special questioning. We had our dog with us and it was hot. We tried to take the dog with us to the building for questioning and an agent said we couldn't. We asked where to leave the dog. He said in the car. My wife said no, it is too hot, and if he wanted to enforce that, he would have to shoot her. Well, he got on the phone and we were allowed to bring our 20 lb dog with us.
Today at the VT border they have
license plate recognition software so that they know who you are, before you reach their station. Last time I went through the agent first words were " Hello Michael".
Mike