I do not know if you could do this, but I was thinking that I could build a rocket outside and run the vent tubes lenthwise under the camper and cover them with a large mass of cob. The area would have to be skirted so no heat would be lost out the sides and probably insulated underneath to avoid loss to the earth below? As the cob heated it would warm the floor of the camper, Maybe?
I need my trailer to be mobile, so this won't work for me. But that don't mean it won't work for you.
Yes, I think the earth will suck all your heat out. Science wise, as far as I know, earth works as a massive "heat sink". So you would want to insulate from the ground good to be sure. You would most likely also want to take any insulation out from the bottom of your trailer, since this would block the heat. Of course, some trailers don't even have this insulation to begin with. In which case, might be a good idea to do the insulating skirt anyway. However, heat rises so the bottom of your trailer having insulation is not as important as the top and sides are. That's why some manufacturers don't even bother with it. My trailer has insulation, it also has covers over the insulation, making it difficult to remove it.