The hills on I-90 are all doable - with a few long pulls - some with rest stops at the top (there is one on top of the 7 mile steep grade coming off the Columbia River Gorge at Vantage, WA).
If you want to avoid the hills, head across I-90 past Spokane, then take 395 down to Pasco/Kennewick, then South on I-82 to Hermiston, OR. Turn right just across the river in Oregon and follow the Columbia River downhill to Portland, OR on I-84 (on the Oregon side). Then back up I-5 to the JBLM/base. It will be a few hours longer, but much lower hills. All but 395 should be 4-lane. It's been a long time since I've been on 395... it might be 4-lane by now, too, but that will proabably be the worst part of the trip - two-lane across the rolling hills of the Palouse wheat fields. The "scenic" route...
And you'll still want to avoid traveling on Sunday afternoon... I-5 between Portland and Seattle can get very, very slow... the rest should be a breeze.
And... Oregon doesn't let you pump your own gas. But, there are lots of truck stops on I-84.