Google maps has most of the information that you need about drop offs, guard rails, shoulders etc. Start by looking the route in terrain mode. This will show where the road crosses flat open spaces, when it's in ravines and where it is winding down or up a cliff. Then there's sat view where you can judge shoulder widths. And finally streetview where you can count guardrail posts.
I can't find the drop offs that the directory warns about. On the descent to the east after the Grand Canyon park turn off there are sweeping curves with long views out toward, but no sharp drop offs. Not much of a shoulder; the road surface looks like it's been around for some time. There are some guard rails.
At the Colorado Rive crossing, Marble Canyon, there are 2 bridges, an new wide one, and a narrow pedestrian only one - the original. US89 through Page has wider shoulders, and may be preferable if you have a big rig, and don't need to see the North Rim.
Speaking of drop offs, US89 has a major rim climb just after the 89A split (east of the river). Several years ago that was closed by a major rock slide. We followed that closure, the detours and repairs on this forum.
US89A closer to Sedona is less-rv friendly, with tighter curves and drop offs. Again that's quite evident on the maps.