You can do it in one day, if you want. I did and it was my first time pulling the trailer and any trailer (other than a much smaller enclosed one many years ago). And in a brand new truck we'd had less than a month.
My orientation was at 8AM. I left Seatac at 4AM and arrived there an hour early. Sat around in the Tim Horton's parking lot as I was too pumped to try to take a nap.
Easy crossing at the border due to being Black Friday, but it's usually a fairly quick crossing anyway. Knock on wood.
Getting off where you are, you'll miss almost all of the Seattle traffic, which can really booger up your time frame.
I did take a nap at the first rest stop after leaving Bellingham. I took some pillows and blankets with me. Tossed and turned and snoozed a bit for about an hour, felt refreshed and made it home the same day.
So, yeah, if I can do it, you probably can as well. And I got to sit in the parking lot of I5 (on the way home) while admiring the Space Needle, Queen Anne and Lake Union.