I've ridden from San Diego to Houston and the only time I was on the freeway was when I detoured up to Flagstaff to have lunch with a friend and decided to make up some time by riding 120 miles or so east on I-40. I didn't have to take I-40 and hadn't originally planned to do so but since I decided to go through Sedona and take the canyon up to Flagstaff it seemed like a good idea to make up a little of the lost time. I've ridden from Breckenridge, CO to Houston, TX without setting even a tire on any interstate. From Houston to Washington DC and back without the need for interstate travel. I did occasionally jump on one when I needed to make time such as after a 2 hour delay due to a logging truck/minivan accident closed US 50 in West Virginia but I didn't have to do so. Found lots of really pretty state roads not to mention Natchez Trace and Blue Ridge Parkway. Another trip was Breckenridge to Lake Geneva, WI, around the southern part of Lake Michigan taking the Lake Express Ferry back to WI. That trip saw Rocky Mountain National Park, Crazy Horse Monument, Mt. Rushmore, Sturgis, Badlands, Walls Drug, Theodore Roosevelt National Park, Yellowstone and Grand Tetons. No interstate required though I'll admit that parts of the Dakotas, Nebraska and Iowa were, well pretty boring but easily doable on a 125-150cc scoot. Though you might want to carry some spare gas. Cross Nevada on US 50 or 6 and you better carry a couple of gallons of spare gas - yeah, did that too from San Luis Obispo through Monterey over through Yosemite. Crossing Nevada on a small scoot is quite possible but there are a few hundred miles of the same type view without any place to stop for food or gas unless you brought it with you then it would be on the side of the road somewhere. There are a few abandoned business/ghost towns with places you can safely get off the road for a break with those consumables you carry. Heck, most of the time you could stop in the middle of the road and have your snack/add gas without anyone appearing before you are finished.
I use Google Maps - avoid highways option to set my basic routes but I'm usually flexible enough that I'll make changes on the fly as appropriate. When time is no issue I've often just set a general direction route on state highways and adjusted as necessary along the way.