San Diego to Duluth - scenic routes?

Rexlion has some great routes and stops, so I would second his itinerary. From Salida, you might want to continue north on US 285 and CO 9 to US 40 at Kremmling. Take US 40 east to Granby, then US 34 for a trip over Trail Ridge Road in Rocky Mountain National Park. Lots of tight curves on Trail Ridge, but your medium-length 5th wheel should handle them if you don't mind driving mountain roads. This route bypasses both Colorado Springs and Denver, so you avoid a lot of heavy traffic.

From RMNP, you could just stay on US 34 to US 6/34 and stay on US highways (not Interstates) all the way across Nebraska. It is a good road and McCook, Minden, and Hastings all have some interesting attractions--a Frank Lloyd Wright house in McCook, a Pioneer Village/car museum in Minden, and an excellent natural history-culture museum and planetarium in Hastings (advertised as "the largest municipal museum between Chicago and Denver").

At Hastings, you can stay on US 34 to Grand Island, then get on US 30 to bypass both Lincoln and Omaha. Much of US 30 is the original Lincoln Highway, the very first transcontinental automobile highway across the US. This 2- and 4-lane highway crosses the Missouri River at Blair, just north of Omaha. From Blair, you can pick any number of US highways across Iowa and Minnesota to reach Duluth.

I have done all of these routes for the same reason as you--to stay off the crowded Interstates (and I-80 is the most truck-crowded E-W one in the West) and to visit some of "small-town America" before it disappears altogether.

Have a great trip!

Once you get to Duluth, I've got lots of suggestions for you.
As I've never traveled overland to/from San Diego...I can't help you there.

It depends on what you like to see and how much time you have.

Hmmm. You could head to Flagstaff and 89 to 160 to 163 and see Monument Valley, the Mexican Hat Rock, Valley of the Gods, and the Twin Rocks at Bluff. Then head east on 162/41 to 160 to 491 into Cortez CO, then follow 145 into the scenic mountains and to Telluride. Then 62 to Ridgway (consider unhooking and taking the truck south to Silverton & back), 550 to Montrose, and 50 east where you can visit Black Canyon. Continue to Salida, then 285 north to 24 east to Pike's Peak, Garden of the Gods, and Colorado Springs.

From there the plains scenery gets a little bland IMO until you get into MN. But coming back you could hit Mount Rushmore, Custer SP, and Devil's Tower on your way to the Tetons. Yellowstone will be crowded, but the Grand Tetons are not nearly as busy (campsites can be found without reservations) and the mountain views are very worthwhile. Heading south from there past SLC, you could follow 89 south where it somewhat parallels the expressway. Or take US-6 west to visit Great Basin NP, then 93 south to see Cathedral Gorge SP.

Maybe these locations sound good to you, or maybe not. You can always get a road atlas and pinpoint places you'd like to see, then look for the scenic highways (indicated by dotted lines) that will connect these places.

I'm going to Yellowstone at the end of this month.
We're just heading up the 15 most of the way, can't imagine taking sideroads across the US.

We are stopping at 3 different sand dunes to ride our dirt bikes, one of which still has wild horses running them some times.

How much time do you have? I just did a Google map search and the sites on the way are almost endless. I would take a more southern route out, then across North Dakota to Yellowstone/Tetons on the return.