One thing I've noticed about Google maps, when you select "avoid highways", the system will undergo contortions to do that. I've had it switch me from one side of an interstate on frontage roads to the other five times in two miles, just to avoid that interstate. What I've taken to doing during planning is to use Google maps to put in my beginning and end points, then gradually zoom in to where surface or secondary roads turn up. You can then drag your route to those roads.
Another thing I've found, and used successfully several times: If I'm in a town overnight on my way from somewhere to somewhere else, I'll go to a local pub and eat at the bar. Striking up a conversation with locals and asking what are good roads to drive has turned up some very interesting things. One time in Wyoming I did this and was sent on a road that actually didn't show up on my paper map at all. Got to a town of about 1600 people and there was an airport - with a half dozen or so bizjets! WTF! I normally don't equate a small town way hell and gone from anywhere, and bizjets. I stopped for fuel and asked about that, and was told that this town (I think it was Saratoga, WY) was sort of an unofficial center for second properties of the ultra-rich from Denver, who would fly up for a couple of days.