Thank you both for the response! I have the bus it is a 2001 33' transit style with a 5.9. The plan is to travel around the Americas.
My idea for the mass was to do gravel or some other kind of rock in a frame. I'm thinking something like hardware cloth in a welded 1" angle iron frame or possibly a lumber frame. I should be able to mitigate the weight by putting it over the rear axle with the water tanks on the opposite side. I have seen people turn these into toy haulers and carry big lifted rock crawlers on them so the weight should be ok. It's the core material and chimney placement that I'm not sure about.
It seems like a metal core would hold up to the vibrations and flexing better but from what I've seen they don't hold up to the heat and burn out fairly quickly. But a good insulated core like I see in Paul's videos doesn't seem like it would hold up to the movement.
Obviously I wouldn't be burning while the bus is moving but I don't know if running the chimney up through the roof would work. I don't know what the air path through the roof would do while moving. But I guess I could build a way to close it while moving.