You didn't say how much time you have for this trip and that might be a consideration. However, we successfully traveled 16 years of full-timing and rarely got on an interstate.
Get yourself a good road atlas and pour over possible routes. Get a highlighter pen and mark the route. You can easily bypass big cities by not making big detours.
Once you get beyond the Plains states I'd suggest you get the 'Mountain Directory of Truckers and RVers' so you don't get into trouble in the mountains. There are many mountain routes that are very easy for RVers though so don't try to avoid mountains. That's where you'll find the beauty. Get the book from the original site, not Amazon. Amazon sells it for waaaaay more than the site does, for some reason.
Good travels to you.
