Earth Sheltered Caravan - suggestions please!

EarthVan - warm air ducted from polytunnel/conservatory to heat sinks beneath caravan

We are hoping to get our caravan out of the barn here soon onto a patch of land beside the orchard. Then we are planning to use some of Mike Reynold's Earthship ideas amongst others to keep warm - it's the EarthVan. The ground will have to be levelled anyway so we thought we could do a bit more digging and make heat sinks to store warm air harvested from a polytunnel/conservatory/greenhouse structure to the front of the caravan.

HEAT SINKS, HELP PLEASE...
How big should we make the heat sinks? And what's the best material to use for them?  Rubble? Bottles? If anyone has any experience of anything like this any tips would be welcome.

FANS & DUCTS
Also, we are planning to use fans to duct the air from the top of the polytunnel section down into the heat sinks, powered by a PV panel - any tips on what specifications for the fans, panel and ducts would work best would be greatly appreciated.

THANK YOU!