The substance you are referring to is potassium chlorate, not potassium chloride. Potassium chloride is largely analogous to sodium chloride, and does very little when heated!
The decomposition of potassium chlorate is as follows:
So for every 2 moles of potassium chlorate you obtain 3 moles of molecular oxygen gas.
The molar mass of potassium chlorate is 122.55 g/mol, so 18,307 g is (18,307 / 122.55) = 149.384 moles.
This will give you (149.384 / 2) x 3 = 224.076 moles of oxygen gas.
At STP this will occupy 22.4 x 224.076 = 5019.302 litres. STP is 760 torr and 273 Kelvin. To get the volume at 39 celcius (312 Kelvin) and 1200 torr pressure, you need to multiply the volume by (760/1200) and by (312/273). Doing all this you end up with a volume of:
5019.302 x 0.633 x 1.143 = 3,631.560 litres.
This assumes that molecular oxygen behaves as an ideal gas, which isn't quite true, but its close enough for this purpose.