I don't think so much that you lose "warmth," but rather, compress the insulation along the folds, Ie, along the creases where the bag has folded onto itself. Which leads to multiple colder spots, due to the thinner/compressed insulation from the compressed/fold areas.

Also remember, temp ratings on sleeping bags aren't absolute. Each company is a bit different. Some use "comfortable at" ratings, some use "you'll stay alive" ratings, some use "good to this temp if you wear a base layer when in the bag" mentality.

OH gets chilly. I'd look at least for a 20 degree bag. Ideally -10 or so, since it rarely gets that cold.

As for storing it, have you thought of a box? Even a 2x2ft box may be OK - it's not "compressed" in its bag, but rather just "cramped." You shouldn't lose too much loft that way. Another option perhaps is stuffing it under a bench seat, if your van still has one (I'm guessing if anything, it's in the far back so your wheelchair can get in).