Whats the best way of navigating the unpaved roads of the US.
Does anyone have any advice on how to find weather they are accessible on a 1200 gs or not?
Found a link about loads of byways in Idaho Idaho but could not tell how challenging they would be for a big adventure bike.

Delorme gazetter is a large print chart with Gps coordinates setup in sections usually of a state and sells for about $14.d has most all county ,state and forestry dirt roads could be cut up along your route and fit into the tank bag window.

There are a lot of unpaved roads in the montana, idaho area some of which I have ridden, usually on a big street bike. have fun and be ready to occasionally turn around and find a better route.
The great divide route goes from canada to mexico and within about 20 miles of my house. Even that route has permutations of harder or easier routes.
I have the most fun on what around here we call "two tracks" this is a jeep or mining road that wanders along, has grass growing in the center and is accessible to four wheel vehicles but not so used that it is badly rutted or dusty.

I have found a very interesting trail called the Magruder corridor from what i can find out about it. Looks very good for a 2 up off road trip

It's true, a good Atlas as listed above will detail the type of road you are looking at. (paved, Forest Service Road, BLM road or Private property) Some are more of a trail and maps should indicate that. I would avoid those. Many roads start out paved, turn to dirt, then can narrow to a Two Track, then go back to pavement. So it can be a mix of conditions.

But most dirt roads in our National Forests and on BLM land (Bureau of Land Management) will be passable, weather permitting. Some are used as Main Roads for locals ... never assume that you won't meet an oncoming Pick up truck ...
going FAST! Or a loaded logging truck doing speeds you won't believe. Use Caution.

You will find State Parks with dirt roads as well. If shown as a road, not a trail, 90% of the time you can make it on a bike if you have good off road skills and aren't too heavy. Snow and Mud change everything.

BLM and Nat. Forests are FEDERAL LANDS and most have roads going through.
Camping is generally free in ALL these areas except in "official" pay camp grounds.

Weather can be an issue. Many dirt roads go through high mountains in very remote areas, most of those roads are not maintained and not cleared by Snow Plows. We have thousands of dirt tracks, especially out West. You could spend years exploring.

Have fun!

The magruder was just done by a guy I know on a GS1200 as well as the primative dirt that parralels highway 12 from lewistown to lolo montana. He had a goooooood time. I tried to ride the magruder last year and got stopped half way thru by snow in june I think.

Research the TAT (trans-America-trail) it is a path coast to coast on dirt.

Also search for the Butler Backcountry maps which will give you a variety of dirt options.

https://www.butlermaps.com/

Last but not least, ask around in the regional forums on advrider. Can't beat local knowledge.

Regional Forums | Adventure Rider

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