Polly, we don't heat ours when on the drive unless we are going somewhere, then we put a convector heater in the evening before (when we turn on the fridge) so that its warmish when we move off. If we are going somewhere with EHU then we take heater with us, otherwise we leave it at home.
We also open all the lockers/cupboard doors so that air moves around and hope we don't suffer condensation, we haven't so far.