I towed our 180 Rpod from Ellensburg over I-90 and down I-5 through Olympia last fall. I'v been towing trailers for over 50 years but after that trip I decided to never tow on I-5 again and it will have to be a really good reason if I ever tow over Snoqualmie on I-90 again.
The traffic is insanely heavy on those highways and everyone is going over the speed limit and changing lanes constantly. I tried to stay in the right lane but at times you have to change lanes...getting out of the Exit Only lane etc...and they wont let you move over.
With you being new to towing I would suggest you avoid the heavy traffic and go I-2. Also, going I-2 you can avoid I-405 and all but a few miles on I-5.