If you use Google maps and input Las Vegas and Puyallup, any of the three route choices that come up would work great.
I hate I-5 with a passion (Especially between Portland and Seattle) so I would choose the shortest route taking US-93 north from Vegas to Twin Falls Idaho, then I-84 in Oregon and I-82 in Washington coming out in Bellevue.
We actually did this route in the opposite direction about a month ago on a 19 day trip to Vegas/Grand Canyon/Mesa Verde/Moab and would gladly drive over that route between Seattle and Vegas anytime. I-84 and I-82 are both great roads which you can make good time on.
There are definitely a few hills along I-84 but the hills are longer and steeper going from North to South than they are the other way. We loved US-93 in Nevada. Nice scenery (very different from the Northwest), well travelled by RV's (saw a tonne of Snowbirds heading North) and a very good road (especially for one with only two lanes).
Some great RV parks and State Parks along that route as well.
2011 Jayco Greyhawk 31 DS