Use Google Maps, bicycle option as a start. Then modify to suit yourself using state maps. Avoid towns >5000 if possible, and of course all interstates. Be aware that US highways usually have a shoulder of some sort, but are often busier than state and county highways with maybe no shoulder. And yes, the ACA maps are excellent.

Make yourselves as visible as possible and ride predictably. That will make the route safer.

Check out CGOAB. Enjoy the planning and anticipation. It's half the fun.