I've made several trips of this type, RV and car, from the plains west to Sierras, and add extra days to my destination trips (a couple hundred over past 45 years) so I can explore new places between NE Oklahoma (and other places I've lived) and frequent destinations like Michigan, Florida, South Texas, Arizona, Chicago. For example, to get from here to Florida it doesn't matter much on distance whether I drive to the Gulf Coast and go east, or the Atlantic Coast and go south.
I have a number of road trip planning resources including scenic road guides from Reader's Digest and National Geographic, three different RV road trip books, additional generic road trip books (particularly like to work with Jamie Jensen's "Road Trip USA" for "what to see" ideas. I also collect state tourist guide, park guides, RV park brochures, guides for major cities. As mentioned, most of this information is now readily accessed online, but I find it easier working from paper.
Since 2004 I've gotten away from national and regional highway maps, putting trips together on Microsoft Streets and Trips, which figures out miles and time for me, works in time for stops (tourist and overnight) and handles major changes quickly and easily.
Staying away from Interstates? Depends on what kinds of places you want to visit. I'm mostly on U.S. numbered highways, I want to see the small towns bypassed by the Interstate system, and we don't have much Interstate coverage in this part of the country. But if your interests are what's in the major cities, the Interstates are the best connection between those cities, and in many cases the older highways are underneath the new roads, or run closely alongside.
