Spearfish South Dakota to Custer

probably the easiest is i-90 east to 16 to custer.

East on I 90 to 385 south through Deadwood, Hill City is easiest. Spearfish Canyon on us 85 is a must see, but the upper canyon to 85 junction with us 14 above Savoy to Cheyenne Crossing is narrow, no shoulders. You can pull through safely, but cant use the pull outs to enjoy the scenery. See the canyon from your TV / towed.
Ask your camp host in Spearfish for local insider info about the sights in the canyon. Spearfish Falls ( the best! ) are not marked, only locals know how to see them.

X2
Spearfish Canyon is beautiful, but I would avoid with your setup.

The best, most direct route is Hwy 85 south off of I-90 to Hwy 385 south to Custer. Both 85 and 385 are good roads and should provide for easy towing.

We were in the Black Hills in September with our 5th wheel.