What rules for Canadian pickup towing utility trailer in US?

it varies by state, but in many states the weight scales say commercial vehicles over XXX lbs. often 20K lbs, so in that case you should be able to just pass by.

Otherwise, I'd suggest at least pulling into at least the first open station you see in a state, it will avoid the possibility of being pulled over as you pass the station by a patrol car looking for trucks bypassing the scale.