As they depend on the sun for heating, you can still heat your shower bag with other techniques. Some ways are mentioned below:
Use a Dark or Insulated Surface: You can place the solar shower on a dark surface that absorbs more heat to warm the water. Asphalt or gravel make good options for this purpose. You may also use the dark surfaces on your vehicle.
Heat it Over a Stove: You can also boil some water on a stove to instantly heat it.