Many of our modern highways do indeed follow the old trail routes and in a few places, like at visitor centers you can catch a glimpse of some old ruts on thier videos. But,for the most part visible ruts are in remote regions out in pastures or long ago covered by sprawling cities.
We actually live a few feet away from the Sidney-Deadwood Trail route. My family homesteaded this land where a relay station and road house once were 10 years after the trail became obsolete because the railroad came to the region.
Our road, which is mostly right on or next to the multiple tracks of ruts is not safe for an RV. Very few of the original ruts are visible from the highway, near the marker out on the highway where it does cross the original route. Locals know where the remnants are but tourists cannot tell which ruts are more recent ranch trails and older trails from freight wagons and stage coaches. This area has been farmed and ranched for about 125 years so in that time many new "trails" have been made and older ones have been filled in my wind and blowing sand.
I hope you enjoy your adventure driving through the areas of the old trails imagining how it might have looked so long ago.