While my experience isn't really helpful here since my preference runs toward highways with a US designation, it occurs to me that for some routes you might be able to do the opposite. I tend to alter to Adventure Cycling routes to stay on precisely the roads you want to avoid more when they detour to smaller roads. I like to ride days on end on a road like US 90 or US 101. It occurs to me that for some routes you might be able to do the opposite. You might consider the general route that the ACA uses and detour to parallel roads that suit you better. I know that is very possible in some places and less so in others.
There are a couple other options I can think of.
You might look at some journals of trips folks did on the crazy guy on a bike site to get some food for thought.
There is a guy who posts here who has done a ton of back road low traffic touring all over the US and has recommended quite a few possible routes to others. You might dig through some of his old posts. You might find something you like. He posts under the username of jamawani, use the advanced search function on this forum and you can probably turn up some good stuff that fits your criteria from his old posts. Some of it may include fairly detailed advice. As you get a bit closer to an idea of where you are likely to want to go he may be more likely to pipe up with some specific advice.