Howdy, just thought I'd sling in my two 'penneth:
I've quite a bit of camping round Spain and Italy (also a fair bit here in UK) and I have found (for me)a very good solution to this perennial problem. I bought a double duvet and sewed a long zipper along the bottom and side, it will then slide nicely into a single duvet cover. It makes a really comfortable and warm sleeping bag. The reason for this D.I.Y. was that I couldn't find a bag that I felt 'at home' with - so I made it myself. True, you get what you pay for, (and mine was not expensive) but it has served me well. On the road it stores into a smaller space than any other bag I've had because after use I roll it and slide it into a plastic carrier bag, then evacuate the air with a small 12v air pump (run from the 12v plug on my Wing). This may not be everyones ideal solution but inside the vacuumed plastic bag it has never got wet, even thru torrential rain.
Have fun, George (Oletimer)