As confirmation- I've been killing the Rumble Crawlers for 2 hours and I've gotten 4 "chest" keys to drop of which I can only hold 1, but there were no coffer drops for Crawlers's nest. I'm moving on to Helm Beetles. Trying to get my AF for warrior done.
I would not recommend hunting Rumble Crawlers if you are looking for a Crawlers Nest Coffer Key. I have killed many of them and all i got were chest keys, and I've never heard of any other mob dropping drop both types of keys.