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.
The cost to build a rocket mass heater depends greatly on the exact design you settle on as well as your abilities and environment for scrounging.
It would be possible to build a functional RMH entirely of cob which could take zero dollars of cash; this would not be vibration-resistant enough for a traveling unit. If you have a community where good used materials are available, you could probably make a good small RMH for a bus with less than $100 cash. If you want to build a batch box style or need to buy new materials for a good part of it, it could cost several hundred dollars.
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