I travel a lot in a 4x4. I've looked at the FJ Cruiser and it's basically the Toyota version of the Hummer -- it looks neat, but it's a waste of time. It's based on the 4Runner, and you won't get much better mileage than the 4Runner -- the MSN website says the FJ gets 16-19 (city/highway) and that's a dream. I have the Lexus version of the 4Runner (the GX 470) and I get about 15mpg all-around. That's what I would expect you to get from the FJ. Consumer Reports says that the FJ sightlines are restricted front and back, and it has very small windows. Also, it's recommended that you use only premium fuel. It's also smaller inside that the standard 4Runner.
In my research on really practical vehicles for this kind of thing, the vehicles that always come out on top are the Toyota, Mazda and Dodge mini-vans and the Subarus; with the vans you can get everything inside; with the Subarus, you can pull a small trailer. Of course, some people refuse on aesthetic grounds to drive a minivan, so your mileage may vary...
The problem with full-sized vans is that you are actually just buying another pick-up, and you get the same bad mileage. If you buy a long wheel-base Dodge mini-van, and are willing to do a certain amount of work (but not too much, really -- you can get everything you need at Home Depot) you can build racks inside that carry a large amount of equipment, and still leave space in the middle for a custom futon (which most many futon shops can make for you.) Along with a camping stove and either a 12-volt plug-in icebox (available at many big outdoor sports store), you can get quite comfortable overnight in the van. You can also get inverters from any car place to run computers, recharge batteries, etc. (The 4Runner versions mostly come with built-in AC power.) I've never measured them exactly, but I go muskie fishing with a friend who drives a Dodge minivan, and you can lay 8-foot muskie rods flat on the floor, with room left over; so sleeping flat is not a problem, if you can find a flat place to park. We use his van (rather than my 4x4) for fishing because it has so much more space; and we've never really been in danger of being stuck, even when we've gone on some pretty bad backwoods tracks. He pulls a boat trailer with it, with a small boat. However, there are places I would go with the GX470 that would be impossible with a van, because of the center road clearance.
Those huge custom sport van conversions I would expect to get ~ 10 miles per gallon, and they can cost upwards of $60,000.
Big pickups with travel trailers never make financial sense, although I understand that some people like the travel-trailer culture. But a travel trailer will cost you tens of thousands, and you can still find motels where you can stay overnight for $50. That's 200 nights for $10,000, and you only have to come up with $50 at a time...I understand that sometimes it's best to stay overnight at a spot so you can get the camera out at first light, but for those occasions, bagging out in the back of the van would work fine. I actually sleep on the ground, and carry a tent, air mattress and sleeping bag with me. No problem.