I was at Twin Falls Campground for the day (fishing) in July 2019, and stayed here back in June 2018. It's another of the smaller campgrounds with only 5 walk in sites. I usually stay in smaller places or go dispersed style when possible. 

It's a bumpy windy road down to the campground. I just drove through here (Aug 7th, 2019) and I wouldn't take a car down to the campground without high clearance.  Didn't need 4WD to get around but the ruts are deep in certain spots. Please don't try to drag a trailer down.

The sites are FREE, which is alway nice, and 1st come 1st serve. The sites are limited in number, but each site is pretty large. The river access is great. The sound is a little loud when sleeping, if you're here when the river is really flowing.

Decent fly fishing in the bend of the Lewis River. 

The falls itself is pretty. While not exactly twins, there are 2 waterfalls.

There is a clean vault toilet, but no running water. 

Kinda awkward that when parking for day use of the river, you still have to walk through a couple sites to get to the water.