If they were mine, I'd have them in the van with me. I always move all live creatures in the van with me. Dogs, cats, rabbits, birds, fish, whatever. I don't have 50 chickens anymore, though. How many chickens are you moving? My dogs and chickens are usually used to being loose in the yard together, too, unless we just added a new one. Are you going to crate your dogs in the van?
What are the temperatures likely to be when you move? Fairly cool? If you air out the truck before you leave so it's cool and move at night it might work. During the day, unless it was really cold, I'd worry about them overheating.
I don't know how much air space they need, just to have enough oxygen and not get carbon dioxide poisoning on a trip that long. When I pack a U-haul, there's no room left for anything to shift! I'd also stop from time to time, check on them and give them a little fresh air.
I don't know what to think about the chance of any exhaust getting into the back of the truck.
What are the temperatures likely to be when you move? Fairly cool? If you air out the truck before you leave so it's cool and move at night it might work. During the day, unless it was really cold, I'd worry about them overheating.
I don't know how much air space they need, just to have enough oxygen and not get carbon dioxide poisoning on a trip that long. When I pack a U-haul, there's no room left for anything to shift! I'd also stop from time to time, check on them and give them a little fresh air.
I don't know what to think about the chance of any exhaust getting into the back of the truck.