I don't have a listing for an SLS. If this is a front-wheel-drive model, check that the electric radiator fan is turning on. If it is, a common suspect is a leaking cylinder head gasket. Combustion gases can pool under the thermostat causing it to not open. The radiator hoses will not be real hot.
If the hoses are hot and the heater blows hot air, check the radiator for cold spots indicating some cooling tubes are plugged, and check if the cooling fins are rotten and crumbling away. The clue there is the temperature will come down when you run the heater on high.