Wheels, I agree with Jeff_M get a Wiggy's bag and ask for a compressed one (explain your situation and I'll bet they can accommodate you)

Usual disclaimers, never owned one, no connection, etc

That's pretty amazing Jeff. I've never heard anyone make that claim before!

I thought bags (at least down) suffered from the process of being compressed/opened rather than while compressed.
As long as you don't crush it repeatedly while compressed I don't think it should suffer too much.

On the topic of sleeping bags - this is a clever idea.
A half insulated (bottom only) bag, since you are likely to have a down jacket in these conditions anyway, you save half the weight of the bag.

http://www.alpkit.com/shop/cart.php?target=product&product_id=16329&category_id=253

ps If you are in the UK (or Europe) alpkit do a small but well thought out range of gear, mostly for climbing/mountaineering that they design/make themselves. (Just a happy customer etc.)

I never noticed much loss of loft on any of the synthetic bags I had. Not even after they were extremely compressed for packing.

Just remember that everything compresses under your body.
Steel is very conductive too, so sleeping in cars is usually very cold.
If you want to stay warm this means more insulation under you than over you. Think of foam or any other padding that does not compress much. Ensolite pads are great. Blankets certainly help. Even cardboard will help in a pinch, so if you store stuff in boxes the box itself is part of the emergency supplies.

Cars also radiate a lot of heat to the night sky. If you have a way to get an aluminized mylar sheet (space blanket) over you it will help a bit. They are not great for insulation against convection or conduction losses but are good against a lot of the radiation losses.
You lose a lot of heat to radiation out the windows and you lose heat out the windows to convection and conduction too.
Covering the windows will help the vehicle stay warmer inside to.
Those folded sun blocker things (cardboard) work to save heat as well as block it out. You know, the ones they sell you for putting inside your windshield to keep your car from baking in the parking lot.

I apologize for the run on answer.

The weakness of synthetic insulation is that it is never quite as effective as top-quality goose down, which is still considered the gold standard. To get even close in terms of insulation performance per unit weight you have to use some pretty high tech and expensive synthetic constructions and you still fall short.

The weakness of down is that it doesn't handle extended exposure to moisture, chemicals or compaction well. All those are real issues for a sleeping bag that is going to spend a lot of time in a vehicle. Inexpensive synthetics are pretty good around water, chemicals and they can recover from being compacted fairly well.

While synthetics fall short in insulation performance per unit weight weight itself isn't a critical issue for a bag kept in a vehicle. IMO the ability to stand up reasonably well to compaction, moisture (say from a blown window seal) and chemicals makes synthetic the better choice. Better to have something that is a little less efficient but which you can count on than a top-grade unit you can't.

I deliberately let myself get snowed in at 11,000' in my Jeep during one September outing. Was actually a controlled experiment for me.

Had an air mattress, 2 foam pads, 2 sleeping bags and lots of warm clothes.

Cracked the windows for a little ventilation and waited out 2 snowy days . Final night was below zero because the clouds cleared around midnight.

Ice on the outside of the jeep from the snow. Ice on the inside from the condensation. Had to wait for the sunshine to warm things up before I could see out.

One of the bags I used is riding in my newest Jeep as my just-in-case bag. But it was cold at 35F when I bought it in 1983 :-)
I like to have 2 bags when actually camping. More flexible and redundancy is a good thing for assured sleep.

Wheels, I actually have some decent experience with this. I have a 0* synthetic North Face bag that I keep compressed when stored (lets say 11 months out of the year, conservatively). I havent experienced any noticeable drop in performance. The deal with synthetics being compressed for too long is that the insulation material can crimp. However, if you remain with the van, or in another sheltered area, it wouldnt be noticeable. And, it takes years for the fibers to "remember" the crimping. Overall, in my opinion, it isnt something to worry about. Down is a different issue; you spend easily twice as much on a quality down bag, and every little thing you can do to extend its life is worth it.

I would recommend a Kelty synthetic bag. They are inexpensive, and work quite well. Hope this info helps!!

I keep my ECWS sleeping bags in their stuff bags, does that mean they will lose their effectiveness over time? The main reason I do this is that there are three bags in one, and it makes it easier to grap and go should I need to leave in a hurry. My ultimate goal is to have my equipment in a state where it will not take me hours to get everyting together. I am aiming for a "grap and go status".

Scafool touched on this, but I want to expand...

A van is a large area to keep warm without the aid of a heat source. Heat sources all produce differing risks from CO, CO2, fire, et.al.

Insulating the interior of the van will be important.

I am sure everyone has seen the windshield sun visors that are made from what looks like mylar coated bubble wrap. That product is know by the trade name Reflectix, and is commonly used as a radiant barrier in construction retrofits on single family homes. I use it in the windows of my RV when it is in storage, and it drops the inside temps by about 20 degrees F. It stands to reason that one could buy a roll of this material and pre-cut insulation for a vehicle. One could insulate the interior of the van in an emergency, creating a smaller area to heat at the same time.

I know that once I take delivery of my new truck I will be cutting panels of the stuff, labeling them and storing them for use in just such a possible emergency while I am touring Canada this winter.

This product also works well as a field expedient sleep system when combined with duct tape to make a "sleeping bag".

All the usual disclaimers, but: I have used this product as it was designed, and in my own designs... Hasn't failed to satisfy me yet.