If you always take I-10 to I-5 that's worse than hills! :) You have all the Los Angeles traffic.
Take I-10 to 95 north at Quartzsite, AZ to Parker to Lake Havasu to I-40 west to Barstow, CA to Bakersfield then I-5 north to Napa, CA then get over to 101 north to Ocean Shores, WA
Plan a stop at Lake Havasu because once you get on I-40 there isn't much until you get to Bakersfield.
Really, you shouldn't have any problem with a 31' trailer if you want to take I-5 straight up.... in good weather. I hope you're not leaving Tucson now. It's beautiful weather... best in the country. :)