Though if something radioactive is warm enough to warm an area, isn't it going to cook you?
Not at all. A fire would also warm an area, and won't cook you - unless you are stupid enough to crawl into it.
An RTG emits energy as heat. The ionising radiation is all contained inside the unit, and it can't get out unless you open it up. It's safer than a fire - and has the benefits of needing no oxygen and lasting for decades without refuelling.
Radioactivity isn't magic 'kill people stuff'; it behaves in accordance with physical law. The usual isotope in an RTG is
238Pu; Plutonium-238 has a half-life of 87.7 years, power density of about 0.54 watts per gram, and is almost exclusively an alpha emitter, with exceptionally low gamma and neutron radiation levels.
The aluminium outer casing of a
238Pu RTG is easily enough to shield the radioactive components. Indeed, even if it leaked, the radioisotope would need to be inhaled or ingested to be dangerous (although it would be very dangerous if it was). 'Don't eat or breathe Plutonium dust' is the simple safety rule, and is analogous with 'don't stick your hand in the flame' if using fire. All energy sources are dangerous if you are unwary or reckless.
Efficiency is pretty poor with RTGs; an RTG with an electrical output in the 500W range would generate over 5kW of heat, making it an excellent space heater. (In both senses of the word 'space').
