Update for Willow River Fat Biking
I talked to the ranger at the entrance station and he told me that indoor restrooms are available at the entrance station and vault toilets are available near the mountain bike trailhead.
I rode the trails today (Friday) and they were in great shape. I looked like they were groomed Thursday night and they setup firm overnight. However, someone was riding with too much tire pressure and was breaking through the snow crust in a few places. 1.5-4 psi works best on groomed trails. I was riding with 3.5 psi and that was just about perfect for me today.