At that scale you are probably better off building your own solution: a low-power microcontroller with a temperature sensor, a small heating element (search "heating foil"), some voltage sensing with the ADC on the battery and solar panels, and a bit of simple logic.
If you are mostly concerned with low-temperature charging you could even power all that from the charging side only.
At 5w, a simple temperature sensitive switch, 5w resistor, and some wraps of insulation might be enough. If below x temp, just shunt all current to the resistor, otherwise power the charger.
If you are mostly concerned with low-temperature charging you could even power all that from the charging side only.