We've taken our RV quite a bit off-highway rock hounding. Our Garmin just about always shows all the obscure roads out there. It's maps are pretty comprehensive.
However ... the Garmin doesn't always show which road is best or safest to use ... because we can't tell it precisely where we want to go as a destination! In these situations we just use it to give us the name of the roads we're on and the compass direction as we drive so that we can then use a Benchmark atlas book to maybe/hopefully tell us really where we are in relation to the big picture.