I think we have been very lucky with our heating. We have a Truma Ultraheat which can work of gas or mains electric. We almost always use it on gas.
There is a fan fitted to duct air into the cab or into the loo/shower but we find the convected air which comes out of the front of the heater without assistance is easily sufficient to heat the van, although in colder weather the fan helps to mitigate the layering effect. We mainly use the ducted air to warm up the air in the shower – it only takes around 5 minutes.
The heater warms up the van completely independently of the battery in a few minutes. It is even fitted with it’s own alkaline battery for the igniter.
The fan is rated at 0.7A (or 8-9 watts). Our average electric use is 10AH/day, so running the fan for 4 hours would increase this to 12.5AH. So we could go for around 4 days without an electrical charge or running the engine and still keep the battery above 50% charge level.
In the winter we always pull the blinds over the three skylights at night, just to help the insulation. The van windows have blinds and curtains, but there is only a curtain between the living and driving areas. It’s a thick curtain though and we have not had a problem. We did consider an outside windscreen cover but the thought of getting out of a nice warm dry van to put on, or take off and stow a wet screen (and where??) in a storm doesn’t bare thinking about.
The only problem we get is with high winds. Gusts above around 40-50mph will cause the heater flame to blow out. Not a big deal if it is occasional, as it re-ignites automatically, but if it is a pain if it is continuous. We have fitted a flue extension which helps a lot, and it blows out much less if we have the offside of the van facing the wind. But if it is really windy and we need the heater, the only alternatives are to either use the cooker for heat or run the engine, or move to a less exposed position. Using the cooker is usually said to be a bad idea, but personally, I can’t see why putting a pan on the burner, or a pie in the oven will make it safer!! We have a smoke alarm and a CO alarm fitted BTW.
While travelling we switch all of the gas appliances off (for safety of course) The standard engine heater will heat the van even on the coldest day but it takes quite a long time to get up to temperature. After all it is designed to only heat the front seats. The Truma heats up the van in a a few minutes.
Some vans are fitted with diesel heaters which seems a good idea as they are about half the price per KWh compared to Calor in 13Kg bottles. But they need electricity continuously. I believe its around 1.5A. and there is a surge in current as they start up.