What do you mean by warped? Exactly what part is warped? The whole cab?
Not much you can do about warped strene or plastic....Could do a couple of things....simulate a wreck....have a junkyard display etc....but as far as repairing a warped Styrene/plastic part Ive never heard of it.....of course I guess it depends on the extent of the warpage.......
Ron since this thread was dug back up......Ive seen a few camper shells available in older AMT kits....just have to keep your eye open....of course that means buying a kit just for the shell....though you could build the rest of the model....it would have to go without the shell.....you could try scratch building one....
You may want to dig around in some of the aftermarket sites and see if they make any....and check out any of the resin sites you might have marked....