Travel and Winter roads on I84 Idaho, Oregon, Washington

I84 is fairly flat all the way from Salt Lake City to Ontario, Or. Then is becomes very mountainous with multiple passes until you drop down a 10 mile 6% grade into Pendleton, OR close to the Columbia River. Then it is a fairly flat road again until you get through Yakima, WA and then you will start multiple long upward grades until you drop down into Ellensburg, WA where you would pick up I90 into Seattle. Snoqualmie pass on I90 isn't very difficult after the ones you have encountered between Ontario, OR and Pendleton and between Yakima and Ellensburg. The weather can be really nasty in any of those areas. If you are pulling a heavy 5th wheel I would recommend taking an alternate route by going from Ontario, OR to Bend, OR and then head North on 97 to Madras, OR then head NW towards Mt Hood into the Eastern Suburbs of Portland and pick up I5 North to Seattle. Longer but much better winter road conditions and many less passes. The only area where road conditions might get dicey in the winter would be around Mt Hood. Watching the weather forecast would be an absolute must no matter which way you go, Once you get into Western, WA you typically will only be dealing with rainy weather but the temps can drop below freezing there too and the roads can become treacherous with ice and snow but not nearly as often as the mountain passes. .