Can I use sand or gravel as mass for a mass heater under an RV?

I have just purchased a 35 ft rv and a small lot. I want to have an exterior kitchen with a rocket stove in the future. My dream is to have the stove vent to a mass heater under the length of the trailer. I will be skirting the trailer prior to the stove and would like to have the mass heater existing before I skirt it in. I envision running a single pipe down the length of the trailer. (I believe an elbow and running it back is to long a run at 70'ish).

My question is what can I use for mass the cover the ducting? I am hoping I can use sand or gravel? So I can shovel it under and not have to disrupt the trailer. That way I can remove it if the trailer has to go and I can pick away at it over time. Will that be appropriate mass? Any suggestions? Will I be able to heat it this way in your opinion?

First off, welcome to permies!

You might want to try doing a search for Tristan Vitali on here. He did a bunch of work with RMH and a travel trailer on his place in Maine, I think it was. He did use cob, but he had a lot of varied experience with his build.

Thanks a ton! I'll search him

Sand would not be a good mass material, as its millions of tiny airspaces make it somewhat insulating. Sand under the duct would be fine, as long as you insulate under the sand. Gravel and rocks around and over the duct are decent if not great mass material. You would want to seal the whole thing airtight to the bottom of the RV so the wind can't blow through and rob the heat.

Tristan's threads  here and here.