In my 24 footer, I use Mr Heater on a propane tank like what Falcon show's, but the smaller one. As with any unvented propane heater, it will give off a smell, create moisture in the air, and consume oxygen from inside the trailer, but you will be opening the side door, and the back drop down gate enough to help with that. I turn it on enough to take the chill out, which helps when working on the sleds. Anything else might be to costly to install or use. I've been in trailers' that use one of those ventless wall hanging heaters from Menards, and they get pretty warm in there, but I can also smell it too.
Switch to grass racing, and you wont have to worry about heat!
Where and what ya racin?
Good Luck,
Kev