The mapping services are poor, especially when I compare them to experience and a little time spent with an (recommended) atlas marketed for truck drivers. Truck routes are highlighted in yellow (and usually mean that the state ensures wide lanes, very good to excellent signage and adequate shoulder prep to keep an 80,000-lb rig from getting stuck.
These non-Interstate roads are often the best route for best time. Draw a straight line from origin to destination at that state or US map with a ruler and examine it to avoid major metro areas, etc. Use the better bridges over major rivers and mountains that the Interstates provide; and avoid anything marked "scenic" (green dots on my 2005 Rand McNally) so as to avoid traffic tie-ups (I have yet to hear of a state that does not make illegal the blocking of traffic; "scenic" routes often have few pullouts to allow other traffic to pass). Those roads one is best counseled by other RV'ers and locals.
Plenty of great roads here in Texas, for example. We avoid I-35 when at all possible. The commercial traffic is heavy, but it is the idiot four-wheelers traveling in packs that are the real danger, towing or not. North-South we like 77, 281, 16, 83. There are a few good E-W routes, but for the most part those Interstates are good (with the exception of I-10 from San Antonio eastward).
Have a look at a truckers atlas (it also publishes clearance heights) next time you are at a bookstore or truck stop; examine a state you are familiar with. Truck drivers assume a planning speed of 50 mph for all miles/time spent driving/fueling/rest areas . . so a few side roads are just the ticket even in cross-continent driving.
And be sure to go to the US DOT map of Road Conditions By State to avoid construction, lane closures etc in your trip planning. Might take me a couple of hours to do a 900-mile trip, but I can keep the time within 15" over that entire stretch with stops every two hours and re-fuel/eat every four. Rested, relaxed and never above 68 mph (with 2-drivers; about 16 hours as I recall; solo). Most importantly, never hit the brakes once except as exiting Interstate to ramp control speed.
RV'ng is still nicest at "300-miles or 3 o'clock".
On the other hand, from a line extending generally NE from Washington, D.C. to Cleveland, all driving is pretty lousy due to sheer numbers. And there are plenty of paved cow paths that MAPQUEST can send you on that you'd never voluntarily run your rig down.