Have you looked at how much room a rolled swag with "bedding and a comfy mattress" occupies? A fair bit more than a self inflating mattress and bedding...
We did actually buy a 2nd hand double swag, camped out in a relatives back yard for a night, Husband didn't like it as he could hardly move to even turn around...yeah it was cosy
, not one of those with the hoops obviously, just a tiny upright pole at the feet and one at the head. We decided to sell it....probably would have suited usage in the tent too, but I think that was before we brought our touring tent anyway.
So yes I realize how bulky they are, obviously wouldn't roll pillows in them and our sleeping bags aren't particularly bulky ones (as much as I'd love to take a doona!). The self inflating mattress I'm talking about is one of those fat double ones which self inflates but has foam inside (4wd ones)....I've noticed skinnier versions in camping stores too. So I still reckon it would take up more room than a swag and I'm sure it weighs as much if not more than a swag!, I reckon the only way to roll it up to camping store size is to use a vacuum cleaner to suck the air out! No way in hell I'm taking the vacuum camping...that's what I go camping to get away from lol.
The problem with having that mattress on the roof is that it would get dusty and wet ..yes I did suggest perhaps a cheaper option would be to get a canvas bag made up for it (or any decent bag) but I don't know he didn't seem keen on that idea for some reason.
Guess that's why most people just have a camper trailer! The Troopy we have is a 5 seater version, draws in the back, fridge, a few other bits of gear and it doesn't really leave enough room for the big mattress. When we went to Albany last weekend for 2 nights I think it was the first time we've camped without having to tow a trailer of some sort (since having the kids)! Must say those hiking mattresses are comfy enough on grassy caravan park ground...for me anyway, but I am the younger half