I can't give you a reply from experience but I've been dreaming of building a tiny home on wheels with cob (maybe far-fetched but we all must dream). Here are a few of the resources I've bookmarked that may help you. In my opinion, the worst case scenario with the cob in a mass heater in a bus would be a few cracks.
Vibration is an issue, but so is chassis flex (not sure how much that happens in buses, it does happen in trucks). How were you thinking of attaching the RMH?
I'm not sure earthquakes are a good comparison. It's the continual vibration over time that is the issue - imagine driving for 5 hours, and then how many times that happens in a year. You could test this - make up something from cob, secure it to the floor, and take it for a drive.
Weight might be an issue too, depending on what materials and design you use.