I think we have been very lucky with our heating. We have a Truma Ultraheat which can work of gas or mains electric. We only 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 sufficient to heat the van even in sub zero temperatures. We only use the ducted air to warm up the air in the shower – it only takes around 5 minutes. So when used on gas it warms up the van completely independently of the battery. It is even fitted with it’s own alkaline battery for the igniter.
However as it relies on convection there is a layering effect in the van so that it is a a lot warmer near the top of the van than at floor. But as there are two sofas, we usually sit with our feet up anyway. Warm slippers are a good idea – I wear Ugg boots – very cosy.
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 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 occassional, 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, 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.