I was about to suggest a pair of stretchers, actually. I reckon a stretcher with a self-inflating mattress on top would be excellent. If you're concerned about the space, you can get little 2cm thick self-inflaters that the hiking guys use. I probably wouldn't like to sleep on one of them on the ground, but one of them on a stretcher would be cool. Alternatively, just a couple of stretchers by themselves. Remember, you won't be dealing with snow up here. A stretcher would give you somewhere to sit on the end of as well, if feeding bub at night. Another thing to pack/unpack every day though.
I certainly wouldn't entertain a self-inflating double mattress of any type - we've got 2 x king single mats, and they're good - MUCH easier to roll up than a single big mattress, and you've got options for storing them. They're nice and light, so as long as it doesn't look like rain, I throw ours up on the roofrack in their thin little bags. If you had a roof pod/canvas travel bag up there, it would be perfect for them.
Is the Trackabout really that much effort to set up? One of the reasons I got a camper was because it as so much easier to set up camp. OK, you can throw up a tent in a few minutes, or 30 seconds if you have an Oz Tent (I have a Microfast so it's one minute) but then you've got to pull out the stove, the table, the chairs, put up some shade, get junior out of the sun/rain/mozzies etc.
+1, I've got to admit. I can set our camper up in about 10 mins (without the annex, which you wouldn't use for overnight stops), including gas, kitchen, etc. The Tourer tent we've got is probably faster, but then as Jules says, you need to set up the bedding, sort out the kitchen stuff, and so forth. I bought the camper so I could drag it with me everywhere, I see no reason not to haul it up the cape. Heck, if McGirr can do it, I figure I've got half a shot
Thanks!
Matto