I have a camping gas heater. I use it if it is really cold for about 10 mins. Point it into my sons sleeping pod (without him in there) so it takes the edge off the cold before he gets into bed. I would never use either that, or my camping stove without the porch open.
I would recommend anyone using anything gas purchases a carbon monoxide detector ( better to wake the whole campsite, than not wake up)
I think the stones idea is fantastic, never thought of that before.