We find a normal indoor duvet works perfectly well in the van and that's what Mrs VD specifies for Cali-travels - including up in the Hebrides where we're touring at the moment (okay I know it's not winter yet).
But if you decide to go for sleeping bags, the main decision is then either down-filled (extremely warm, until you get it wet) or synthetic (less warm than down weight-for-weight but usually cheaper and better if it's likely to get damp).
By the way, don't be fooled by the temperature ratings of sleeping bags. When it says (say) -10 degrees, that will invariably mean that's the coldest outside temperature for getting any sleep at all - or even just avoiding hypothermia. You won't feel comfortable and snug at that temperature.