ChickenNannie,
You may already know these facts, but I'll post them just in case you don't.
Salatin first used a trailer (with tires) on which he built a 12'x20' hen house. The coop itself extends over the end of the trailer bed. He liked that one, so built the same thing again. They are connected with a very strong hitch, and he moves 700 laying hens in the 2-part EggMobile.
So first, you need a trailer with decent tires.
And then I would just play around with paper/pen to see with what you can dream up. The top several feet of the pen is open all the way around for ventilation and light. This would need to be covered with strong wire to keep the varmints out. Salatin has panels that are attached at the bottom of this window opening with hinges to come out and down. During nasty weather, the panels can be swung back up into position and locked.
The roof looks flat, however I would think of a shed style; this will be nice if you have to be out in the pouring rain. The rain can be directed away from the people/chicken doors.
The nesting boxes are lined up on one wall, and are accessible from outside. Salatin provides 48 nest boxes per trailer. That's seven chickens per nest. (Or more if they get the notion in their heads they all want the same nest.) You can stack the nests, just be sure there is a 'roost' in front of the box so the hen can easily step into it. Think of how you want to open the nest box door. It has to be strong enough to handle the constant opening/closing. But not too heavy for you to open. You may want to have this hinged at the bottom, so you don't need to worry about it coming down on a hand. Several access door are better than one heavy door.
You could put roosts on the other side of the building, with a pull out tray located under the roosts. This would keep the floor of your Eggmobile much cleaner. Feed can go in covered trashcans inside the hen house. Use a can height with which you can easily reach the bottom. Water and feed could be kept in one or two or three areas..... all depends on how many chickens you have.
There is a good sized chicken door with ramp at one end. Again, according to how many hens you'll have. If you've got 300, it may take them quite a while to leave in the morning single file!
You'll need a people door, also. If you put that more in the middle, it will be easier to access either side of the trailer as you need.
Salatin uses wooden floors with chicken wire over them. You want to have a floor that predators cannot access, just as in any other coop. You could use a deep litter method, but that might get a little pricey, depending on the size of the coop. You might want to look into finding some cheap linoleum.
Ask this forum what size posts and plywood for your walls you may want to use. Because the trailer is your main support, you perhaps can use a small post. Or not.
Anyway, these are just some thoughts to get you going.
I'll bet with BYC to answer all your questions, you can easily have your own Eggmobile, and more to your own liking.
You can call it The NannyMobile.
Carla