I've done that trip many times, both directions about once a year for the last 10+ years, both in a class C with toad as well as with bumper pull trailers. It is my favorite route. I avoid Tacoma/Seattle traffic as well as that 10 mile pull up from Vantage when going West. A bit lacking in rest stops between Yakima & I-5 but if your bladder makes it to the top of White Pass the gas station has public rest rooms. The only official rest stop is a couple piles west of Packwood.
When heading west I top off the tank at the King truck stop just before the Tri-Cities on 395 about a mile before heading west onto 12. At 8 mpg with a 50 gal tank that got the motor home to I-5 with plenty to spare.