I'm trying to get the same. I would like to get all event in a room calendar selected on the calendar dropdown select. I've been trying to change some values on the dropdown select and I get all rooms list. However I'm not able to get the events from that room calendar (I want to get all events, not only the events related to my calendar). I've done all changes proposed in this post and I didn't get any resoult.
This is all I have been able to achieve similar to @emadrazo. The dropdown selection provides a list of the rooms, however no events show in each calendar as it seems the calendar isn't linked to anything. However, when I tried the method before it still only broght up my calendar.