It seems that you have crossed purposes there.
You do NOT want to "see cities" by taking a motorhome through them on a secondary road.
But between cities, many of the major Interstates have US routes that run roughly parallel that usually are scenic, in good condition and with almost no traffic. Without looking, US 40 and 50 come to mind going east and west.
Use your favorite electronic trip planner or GPS and set it for "avoid highways" and see what it suggests. Then when you get close to a major city, start it over again allowing "highways" to get you through or around town better.
Note: Unless you have a GPS that will route specifically for RVs or trucks, pay attention to the route it selects. In my experience it often will pick some "short cuts" that aren't the best.
