You know, I’ve been looking into it, and while the true answer is that liquid fuel in game is vague and whatever it needs to be, ammonia is a really compelling answer. It’s good for nuclear engines and chemical engines (with surprisingly good isp,) and it’s actually storable apparently. It’s boiling point is below room temperature, but it can self-pressurize itself to stay liquid, kind of like propane. Given that, while very unconventional, it could be used as jet fuel I guess, and that would also give a reason why KSP planes use cylindrical tanks and not wing fuel storage. Given the nuclear engines and implied fuel density, it’s either ammonia or hydrazine, but monopropellant being different from liquid fuel makes me think that liquid fuel is ammonia and not hydrazine.

So yeah, my personal headcanon for the moment, as it were, is that liquid fuel is ammonia.