Well there's enough stats here.
Suffice to say there's many dubious quotes about the fuel economy of the diesel.... people claiming 30%+ better than factory specs. It's one of my pet peeves.
AS a rule of thumb though, you should be looking at about (rough estimates here from previous threads).
LPG (dual fuel) 4.2 25l/100km (town) 20l/100km (highway)
Petrol or straight LPG 4.2 20 - 25l/100km (town) 16-18l/100km (highway)
3 litre petrol 16-20l/100km (town) 14l/100km (highway)
EFI Petrol 4.2 18-22l/100km (town) 16l/100km (highway)
Diesel 16l/100km (town) 12l/100km (highway)
From what I've seen, there seems to be little difference in fuel economy on the TD's, although obviously a lot more go.
As everyone has different tyres, tyre pressures, lift kits, carries different amounts of crap, roofracks, trailers, travels at 10kph over or 10kph under it's all pretty variable. Big muddies knock sh1t out of your fuel economy too, and out of your speedo accuracy to boot.
On the upside, putting huge amounts of weight for touring doesnt really hurt fuel economy, nor does towing a trailer so long as it's not some big windsock. Even towing a loaded 8x4 behind my SWB I could barely feel it.
Heavy offroad work can see 50l/100km out of a Petrol and high 30's at least out of the Diesel. Make sure you take that into Cookie when looking at safe fuel ranges.
Certainly the diesel is going to be noticably cheaper to run than the 4.2 petrol, it wouldnt work out much if any cheaper to run than a 3.0, and would cost way more than a LPG 4.2.
If you intend to do a lot of distance trips in the outback diesel is really your only option, although a good dual fuel setup should comfortably allow something around or over 250 litres in the LWB. You'd be suprised at how far that means you can go before you need to switch to petrol.
Even town based 4wd's like Prados have 160 litre tanks standard....
LPG will use almost twice the fuel of a diesel, but it's about 1/3 the price. Plus the $5k premium that people ask for on a diesel pays for years of fuel for many people....
If you're talking about a couple of one-off-trips some remote time in the future (long service leave or the big dream) then I'd be going the dual fuel. If you ever do actually get to go, you'll probably end up towing a trailer anyway, so you can carry some jerry cans.
After all you're going to want to take hundreds of litres of water, at least 2 spare tyres, spares, chainsaw, recovery gear, tools and all that normal camping stuff as well. Unless you're in a convoy with quite a few vehicles, that means trailers, or big loads on roofracks.