I don't think it's sensible to recommend you to do this. If you became ill (or worse), you would be perfectly within your rights to blame anyone who gave you contrary advice.
That's not to say I have never used a heater inside a tent myself!
I just wouldn't reccommend it. See link
http://www.campingandcaravanningclub.co ... poisoning/
If you have a tilley lamp or better still a Coleman multi-fuel, they give off both light and heat. - but I still can't recommend this even though I have used them myself - for the same reason.
(If you have a sewn in ground sheet there is an additional risk from escaped gas pooling in the bottom of the tent.)
Any heat source near canvas is asking for trouble too. It takes seconds for the lot to go up in flames and any plastic bits (often in the roof) will melt and drip.
https://www.youtube.com/watch?v=jeQS8DFfU2Q
1990 Autosleeper Trident. Water cooled 1.9L DG + 4 speed manual box