Kinda hard to imagine what you've got in mind completely so my thoughts are only based on what I think you said (how's that for a disclaimer?
Having raised, ridden, and shown horses for many years I'm pretty familiar with bucking bales of hay...and they're heavy!. It would take a fairly substantial porch to accommodate the weight of very many bales and it would also take a fairly substantial porch to tolerate the hitch weight from a loaded horse trailer. I don't see this as being anything too difficult to do but since you'll have a lot at stake doing some basic load and stress analysis would be well worth it. I don't see how you can do the folding thing with hauling hay and pulling a trailer in mind. The deck can certainly fold easily enough but it seems to me the support structure will have to be quite strong and permanent (which would defeat the purpose of a folding deck).
A folding deck just as a place to hang out would actually by quite easy; a couple of stout hinges and a couple of good cables and anchor points and you'd be all set.
Personally, I'd want a very substantial hitch attached directly to the frame of the bus to tow a horse trailer. My 2-horse trailer weighed in at over 5,000 pounds with both horses on board and normal gear in the tack compartment; tongue weight was around 700 pounds. That's not a load for a wimpy set up! And since that wasn't just some replaceable travel trailer tagging along back there but a trailer hauling live animals I cared about deeply I was especially cautious about everything being perfect. In thousands of miles of towing I had no troubles at all.
My belief is that done right adding a porch to the bus capable of dealing with the weight of hay and the tongue weight of a trailer would cost far more than buying a horse trailer with hay storage. You'd also put even more weight further aft of the bus' rear axle which could (and I emphasize could) have implications on how the bus handles and even more overhang would mean more times when you might bottom out the tail of the porch.
I'd personally opt for the previous mentioned heck-for-stout hitch mounted right to the bus frame with a folding porch on the back just for being lazy and a trailer with some hay storage.
Also, especially if your bus is diesel (I can't remember), I'd look into doing something about the exhaust. When I tow my boat with my Blue Bird it gets coated with a pretty thick film of diesel exhaust. That is not something I'd want my horses breathing. It's especially bad when climbing hills (lots of fuel going through the engine) and since the speed drops substantially the exhaust follows the bus more and rolls in behind it...right where the trailer is.
Best wishes on your project; this isn't meant to dismay you but to bring up some questions for you to ponder while you decide what will work for best for you.