We've got 2009-2010 (non DEF engines) Thomas Built EFs with Cummins 6.7L engines. The coolant overflow rez is in the driver's side side wiper compartment. I'm unsure as to how much coolant is supposed to be in the rez. I keep the fluid level at just under a half.
All three buses lose coolant. The rezevior is empty about once a week. The oil is not gooey, there is nothing visible under the bus, but its going somewhere...