In my last trailer, a 30'+ 5th wheel I had no working heat so I'd run my Mr Heater Little Buddy on a 1lb tank. Ended up poisoning my plants. Seeing the oil slick when I watered is how I could tell it was the cause. As soon as I stopped using it they perked back up. These are built to be used in enclosed spaces (not Cali legal use) but they don't burn clean. It was good enough for me when I'd vent the trailer from time to time but I wouldn't recommend them to heat as small grow space.
If possible, get get a unit that exhausts the combustion to the outside and only radiates or uses a blower to heat the room. Better for the plants and probably the humans/pets too.
Someone suggested carbon monoxide detectors, good idea as well. Someone else said they don't produce carbon monoxide, they do. Make sure they are cleaned on a regular basis. From what I can remember, Inefficient/restricted combustion produces higher amounts of carbon monoxide too. Even devices cleared for indoor use can be deadly if not properly maintained.
I'm so glad the little 5th wheel I bought recently has an awesome ducted furnace and is an Alaskan (sub-zero insulated/weatherised)