I am in the same boat
I made several car trips to Canada also. Most of the time, they did not even look at my greencard. Sometimes, they just wave me in. I have no idea what records they have and what they don't. I don't have any records since there are no stamps on my passport. I am filling out my N-400 now and I plan on looking through my credit card statements to see if I spent any money in Canada and use those weekends as approx trip dates in my application. I also want to put a disclaimer there.
Is this ok?