Portland, OR to Phoenix

X2 on what mockturtle said, 93 is a good route. How about traveling down the 101 through CA, it's a good road and scenic (it may be a little further). I would prefer that over I 5. Just as you reach Ventura cut over to I 5 on 126, it a scenic route through lots of farmland. South on I 5 for a few miles to the 210, follow that all the way across the top of LA until it drops you onto the 10. OR you can come across I 80 to the 395 south. At Yosemite we went east on the 120 to the 6 into NV to the 95. This route would also require watching weather forecasts.
We were on I 15 in ID & UT one Thanksgiving pulling a 38' 5'r, I would not do it again. I 15 was icy, 6" of snow in Pocatello, never again.