That I-90 to I-84 to I-15 idea is a pretty good choice. I know about Snoqualmie and the climb out of Ellensburg and Pendleton, but any travel outside of Cali is worth it.
We've just returned from Vegas using I-15 and it's not too bad. The route is higher altitude south of SLC, so temps might not be as bad as Cali. Fuel will be cheaper than Cali. Lots of places to over night (except SLC area). There is a Cabelas in Farmington, Ut with RV spaces.
Somehow, I've been on all the afore mentioned highways in the last 3 years and I-5 would be a dismal choice.
Just my thoughts. Have a great trip, stay safe, stay well, and enjoy.
2016 Ram 6.4 Hemi CC 4wd
2019 Timber Ridge 24RKS