Most of them are machine washable these days.
You will need a huge washing machine. Mine isn't big enough to wash a single sleeping bag never mind a double.
In the previous VW camper we had a king size 2 seasons sleeping bag. SWMBO made a bespoke cotton liner out of 2 sheets. It wasn't warm enough when there was frost outside and we ended up wearing fleece jogging tops and bottoms at night. In summer it was too warm so we took the liner out and slept in that. Getting in and out of the sleeping bag was not as easy as with a double duvet which is wnat we now have. 4.5 TOG is enough for us.
Anyone want a king size sleeping bag? Free if you collect it yourself.