Heating/using a caravan in the winter

Hello

New to caravaning ish and new to us we are going away for a couple of nights in December in our challenger 558.

Obviously its cold at the moment and I have played around with the heaters (gas and electric)

I am after the following advice please.

1) Fastest way to heat your van ? (gas/electric with or without blown air)

2) Heating a cold van will cause lots of condensation initially. I understand that airflow helps reduce this but also lets in the cold. Any thoughts ?. 

3) Can I leave heating on when i go to bed or go out so its toasty when i get back ?

Any answers or thoughts appreciated and tips or advice.

In the meantime Im off to buy some slippers as I tried sitting in the van from freezer to warm and my toes are numb 

Thanks 

Blakey