On mine it's a large black can, about 8 inches in diameter, maybe 14" tall, on the left (drivers side). The tag axle adjustment guage and valve are just to the left of the reservoir. On top is a cap about 1 1/4" in diameter with a 4-5" dipstick on the bottom of the cap when you take it off. It says nothing about what it's for on it anywhere, but as Gerald says it holds the fluid for power steering and the hydraulic fan. Inside is a 3 pack of filters which should be changed as required on PM schedule.
RR10S is the tag axle chassis designator for that era of coaches. My owners manual has info on fluids and maintenance on it. I have no separate chassis manual either.
Ken