Last winter we cross over to the East Coast, the opposite direction, and I had two routes planned, I90 and I80 and when the day came to leave we checked the weather and chose I90. Since, like you, we were on no time schedule, we checked the weather every morninh and if it was going to be bad, we planned to sit it out. We were lucky, blue skies all the way.
Your going to have to go over the Rockies any way you go and the I90 in Idaho is the highest elevation that you'll come to. Its not great, flat is great, but its not bad at all.
Make sure you have chains and I was going to say, know how to put them on, but since your from Iowa, I guess you would. (Folks here in WA, not so much)
Also, know that most RV parks are closed in winter up north, so have a plan where to stay. Casinos might be a good option.
Get another heat source for your rig. One of those Little Buddies are wonderful as you can use them in the rig and it keeps it warm just using bottled propane.
Happy travels