On my coach, as well as my previous one, there are multiple vents for the Webasto system. Each heat exchanger (radiator) has a vent at its high point where the coolant enters the cabin of the coach; these vents are accessible through removable panels.
I also found vents on the OTR heating system: one under the dash, as Jim pointed out, and another at the large heat exchanger in the third bay, roadside. When I recently changed the coolant, I bled all these vents, but I still had to add 2 gallons after my first long trip. Clearly, I didn’t get all the air out of the system.