If the camera gear is warm inside and out you will not get condensation forming when you remove it from the top box and case.
Condensation forms when warm moist air hits a cold surface whose temperature is lower than the dew point of the warm air.
The bigger risk is when you put the camera back in the top box, if it is cold and damp outside this, dampness will permeate into the top box, after using the camera outside it will have cooled down, so when you put it back in the top box that is when the risk of condensation forming on the camera. This is because the camera will be cold and the air in the top box warm and moist and so condensation could form.
Keep the top box unheated and if really cold take a spare set of batteries with you.
Oh and when taking the photo be careful of where you breathe when you get the camera out.