Usually it's done with hydraulic rams (Alaskan) or electrical or pneumatic versions of the same thing.
As sarconcepts explains, keeping them synced is the problem when raising/lowering. The main drawback that I can see from that, is that you have to leave a bit more room between the top and bottom to allow for the roof going up not quite square.
Another problem with the hydraulic and pneumatic rams is that you can get "creep" so they often have some sort of block or pin that you put in to keep them from creeping down.
And of course, hydraulic brings the spectre of an oil leak.
I've often thought that a more or less standard machine type worm drive screw jack arrangement might work well. As an example, here are a couple of different arrangements. These are very much unsuitable for what we're talking about, but you'll get the idea immediately:
http://www.nookindustries.com/jack/JackArrangement.cfm
The first obvious problem with that sort of setup, is that the nut remains stationary, while the screw goes up and down. I think that's probably the reverse of what would be needed for a camper roof lift. To see what I mean, here is a link. In the main photo, notice the screw in the center. On that rig, the screw is stationary, while the nut rides up and down:
http://www.concisemotion.com/screwjacks.htm?gclid=CPG2xd6Gz6UCFYHc4Aod3nXAkw
Here's almost exactly what I'm talking about - at the bottom of this page, there are a couple of pics. For a camper roof lift, I really like the look of the last one with the motor in the center of the main shaft (though the rig in the pic is way too burly for a camper roof lift):
http://www.lim-tec.com/english/products4.html
This sort of arrangement could easily be cobbled together by a homebrewer with a couple of shafts, four screw/nut sets, some worm gear sets, two right angle gearboxes and a few bearings (could very likely just get everything from Grainger's). You'd need say one set of worm/ring gears for each corner, and perhaps a 5th set for the motor to drive the main shaft (or 6 - make them all the same and you could carry a single set in the spares box):
http://www.concisemotion.com/wormgear.htm
http://www.tolomatic.com/products/product_detail.cfm?tree_id=141&category=gearboxes-gear-boxes
Then of course, you also need a motor...how about a Bosch VW starter motor? If that thing ever quit working anywhere in the world I would bet you could find a replacement or some shade tree mechanic who could rebuild it on the spot. Or perhaps some small standard type winch motor. If you did it right, you could even add a hand crank to the main drive worm "just in case". It probably wouldn't be all that hard to turn, but you'd have to turn it a whole lot of times. Make the nut the same size as the lugs and you could use your lug wrench to crank the roof up or down in a pinch.
The whole rig could be easily built right into the base frame of the box, and end up only with 4 screws sticking up through the floor. Add a few access covers underneath and put it on the "annual inspection and maintenance" list. Attach the roof to the nuts, put on some top/bottom limit stops and away you go.
A couple of advantages I see right off are that it will certainly be synchronized. And that setup -will- go up and down square no matter what wonky angle the truck is parked at, so you can build the top with a tighter fit to the bottom.
It's relatively simple, yet quite mechanically robust. Bulletproof basically...about the only thing that can fail is the motor. Weight will be irrelevant - even small screw jacks can lift tons. Unequal loading of the roof will be irrelevant as well.
Another advantage that I particularly like, is that wherever the roof is when you release the switch or stop turning the crank - there it stays. It's mechanically -locked- in place. So if you wanted to, you could easily drive around with the roof raised.
Gale force winds? Six feet of snow on the roof? Bah! Piffle! It also can't leak or creep.
Of course, you could just buy all the parts off the shelf. Here's an excellent document that shows how to do the engineering math to figure loads and torques and whatnot, and it also shows some really sweet products:
http://mdmetric.com/prod/pti/Muli.pdf
For the guides - again, standard industrial linear guides should get the job done:
http://www.thomsonlinear.com/website/com/eng/products/linear_bearings_and_guides.php
This one looks sorta sexy. A set of eight - two on each wall - should keep the top perfectly aligned:
http://www.thomsonlinear.com/websit...uides/profilerail/microguide_linear_guide.php