The Cass Scenic Railroad is a fun excursion. I've ridden it twice. The first time I stayed in one of the 'factory workers houses' that have been remodeled so that they are modern on the inside and historic period looking on the outside.
The second time I took the RV but stayed at Seneca Shadows Campground, which is about an hour away and drove the toad there. The road getting there is a bit narrow. I did see a few RV's in their parking lot but I don't think there are any overnight provisions for RV's at the park. We did see a few RV parks on the way that were closer.