Have a look at the Roman Gemini - the two bags can zip together to form a double, each one can be opened up like a quilt or blanket, and it's designed that the two can fit inside each other for extra warmth for a single sleeper.
http://www.kellyscamping.com.au/p/11659 ... -twin.html $119 - remember it's potentially two bags
Pair it with a double self inflating mattress. Approx $120 - shop around
http://www.kellyscamping.com.au/p/97344 ... g-mat.htmlOr a queen self inflating (we got this for our OzTent RV5 on a queen camp stretcher (splits to two singles) - warning - it's pretty big! 12 cm thickness) $229
http://www.ebay.com.au/itm/OZTRAIL-ROYA ... 2a2a274e1eDO NOT get an air mattress. You will regret it.
Other suitable ones might be the Coleman Pilbarra range designed for swags - cotton outer, with removable flannel liner. Comes in 3 different temp ranges 5 deg C, 0 and -3.
This one is $80. Kelly's can have it to you in a couple of days
http://www.kellyscamping.com.au/p/97346 ... ra-0c.htmlAdd a fleece liner for extra snuggliness. $25
http://www.kellyscamping.com.au/p/46793 ... liner.htmlOr others on e-bay (they may be cheaper, but post takes for ever from Hong Kong and the stuff sacks are complete crap)
This is what our kids have for their canvas domed swags for scouts. They use conventional mummy lightweight bags (different summer and winter ones) when they backpack in, but most of their camping is vehicle based.
Wild Earth have a good range of this type of bag too. Their service and postage is very good too.