If you shop outside of a big box store you can find 6" PVC. Any real plumbing supply will have it and larger, but it's probably not the right solution for the problem. It works great to make tanks that fit between the floor joists on an enclosed trailer, but for under the cab of the truck, you won't have enough room for an appreciably sized tank. It'd be more efficient to use gallon milk jugs, LOL.
However, if you can find a suitably sized and shaped tank, I see absolutely no reason not to do it. You may want to add some sort of shield for road debris, but other than that... Why not?
Oh and for those of you who insist on passing judgement for dumping gray water, get off your high horse. I, and everyone I know who camps, dumps their gray water on the ground. Yes I said it, I DUMP MY GRAY WATER. Within 10 minutes you can't even tell anything was dumped. Food particles? Scrape your plates off into the trash before you wash them like normal people. Your camper doesn't have a disposer so food particles should not be going in the sink.