Seems oddly unsafe to have a heater in a craft with a belly fuel tank, but I believe the tank compartment is south of the sleeping quarters, and open to atmosphere area does exist above it. A Carbon Monoxide(CO, not CO2) detector would be handy anytime fossil fuels are burned in a closed compartment. I assume the catalytic heater has a low O2 sensor, similar to portable RV heaters? This is assuming you intend to sleep while the heater is operating. If just to warm up to get into bed, then I don't see any issues. Personally, a nice blanket/sleeping bag is good enough for me, I love 40 degree weather.