I need advice. I'm looking for a killer road trip vehicle. Ideally, it will have:

high clearance, 4WD, and locking rear differential (for rough roads, trails, and beaches in Mexico, Alaska, and in between)
a low rooftop (for easy access of the bikes/kayaks/skis/etc. on the roof rack)
rear seats that fold down flat, and enough room behind the front seats for impromptu sleeping (6'+)
four doors (for when we drag our friends along)
good fuel economy (for long highway hauls)
great reliability (for peace-of-mind when traveling in the backcountry)

My wife and I can scrape together up to $30,000 for something new or used. Whatever we buy will also have to serve as her daily driver. I've considered a crew-cab pick-up with a bed-topper, and looked at a few SUVs, but I've yet to find the perfect solution. Do you have any suggestions, O Wise Ones?

With any 4x4 and a roof rack with stuff strapped to it, you probably won't be able to get 20 mpg. But you can expect 15-18 mpg, generally speaking.

There are no new 4x4's with high ground clearance and a low roofline, unfortuneately... Newer trucks bodies have been getting taller while maintaining the same amount of ground clearance, possibly because the engineers think it looks cool. Just like all the 20 inch rims on new 4x4's, there is absolutely no advantage to them offroad verus smaller rims like 15 and 16 inchers, unless you want to look "gansta", lol.

I don't know of any in particular with fold-down rear seats, but since you wanted reliability, better fuel economy and a locker, a new Toyota crew cab might be the way to go. The Toyota is supposed to be mariginally more more fuel efficient than other trucks, plus in 4wheel and Offroads test drive pages, they test drove all the compact truck 4x4's and the Toyota out-performed them all. (all vehicles in stock form)

Have you looked at the new FJ Cruiser from Toyota? It gets decent gas mileage and you can get it with the rear locker,roof rack and 32" tires. Plus its a Toyota so it will be reliable and wouldn't be a bad daily driver either. I'm not sure if there is 6+ feet of room in the back with the rear seats folded though.

The new Dodge Power Wagon. All locked up, extra cab for more seeting and space for your toys.  

I would have to agree with the Dodge PW.  Its not going to be good w/ gas, but it has everything you need for back country exploration, especially if you are alone, (no other 4x4's).

The real question is are you going to really use it the way you describe??

Thanks for the suggestions so far, but a barebones Power Wagon  MSRP is over $41,000, and only gets 14MPG (w/o a roof rack!)on the highway. I like the idea of a SUV because the rear seating/sleeping area is combined, making for a shorter (and presumedly, a tighter turning) vehicle. Any other suggestions for a SUV w/ great off-road capacity, fuel economy, and enough room to sleep in?  

Nissan xterra is supposed to be a good all round veh for on and off highway.  

Take a look at Range Rover.  

if i was getting a road trip vehicle it would be one of the following

crewcab cummins dodge truck

toyota four runner

toyota fj-80 or fj-60 land cruiser

nissan x-terra

jeep grand cherrokee

jeep cherokee

chevy blazer

hummer h-3

only the x-terra, fj-80, and h-3 come with locking differential in SOME models.

and of those I think the fj-80 would be my choice, even though it probably gets the worst mileage

Friend of mine has an x-terra. cool ride, but sorta cramped compared to my '77 K5. Someone else I know has a Montero, and it's the same deal with that. It don't look as cramped from the outside, but inside your'e up close & personal. - bit too close for my taste.

My K5 would put a hurtin on anyone's wallet on a road trip. I used to wheel mine to Key West from Fort Liquordale years back - about 6 hours each way.. had to stop about 6 times total for gas. - gas used to be a whole lot cheaper back then too!

The Yota's are pretty reliable from my experience too.

The only thing I ever had to change on road trips on my K5 was the mechanical factory fuel pump. I went with a Holley HP pump since, and never had a breakdown problem again, so far. I carry one with - just to be on the safe side though. These pumps usually give you a warning - slosshy gas pedal when they're about to go. I don't think the electrics give you the same 'grace period' to get somewhere down the road, from what I've heard. Once it dies, it's ded!

I'd go with a plain 3/4 ton 4x crew cab dodge...the powerwagon set up is nice but not needed...my one ton crew can get 18mpg with out any add ons and my buddy with his single rear wheel one ton got 20mpg stock at 55 mpg towing his camper...a simple reprogrammer could add enough hp/torque to squeeze a few more mpg out of it and still have enough power to pull a trailer load of kayaks, gear etc....a topper on the truck would give you a nice size area for a bed and we put cargo nets in the top of our topper to store all sorts of little stuff in...IIRC you can get after market fuel tanks to store 50 gallons instead of the stock 35...I suppose you could put stuff on top of the topper but that would surely kill fuel numbers...nobody ever wants a little trailer...??? I like them...oh..I just priced out a new dodge 3/4 crew 4x CTD on the dodge web site...$44k...a little more than you wanted to spend..maybe find one a couple years old