We have driven FM170 in a 36ftMH towing an F150 several times. From Terlingua to Presido it is a 2 lane road, has a 13% grade up a few miles west of Lajitas, then 13% down, with several switchbacks in both directions. There is a short rock wall on the river side. Not nearly as scary as the Bear Tooth highway.
This road is mostly switchbacks, think 35 - 45 mph. With that in mind, it is the main route for trucks between Presidio and the National Park, unless you go up through Marfa and Alpine.
It is very doable with an RV, nice scenery, you are following the edge of the Big Bend Ranch SP and much of the road follows the Rio Grande River, do have to watch for donkeys and horses on the road. Loads of places to pull over and take pictures.
That is one of my favorite places, go and enjoy driving the river road.