Oh, and about using the trailer for storage... Any trailer which transports animals will have lots of ventilation, and if you get stormy weather, horizontal rain etc like we do, hay would get wet if left in a trailer in the field, unless you cover all the ventilation gaps. (And tying tarps on creates more things for sheep to damage... lol)
I've done it using an old Ifor TA510 on some remote moorland where the difficulty was lack of access after the slightest rain. It worked, but with some issues. And in this situation it meant waiting for the weather to dry up before moving it back out so "tied it up". But it wasn't a bad idea for using an old trailer, and saved not just mauling onto site, but also mauling out of trailer to store and then feeding out to racks in due course.
Yes, any cables, including those underneath will get nibbled, side marker lamps especially. This was an old one so they didn't work anyhow. They will also rub against edges and mudguards. You need a fence around it. You also need to be on a good surface, as it will sink into a field with the weight over an extended period and make pulling it out a bit of an effort, also ensure the underside is not on long grass that will rot it.
I can't comment on horse trailers, but an aluminium stock trailer will give condensation inside the roof that will drop onto the hay and rot it. You can put a tarp over the hay but you'll still get (lesser) condensation under that. Similarly for the sides, don't cram them in against the sides and front/ back but drape the tarp over them to stop ingress from vents even the flap ones shut, and allow some space underneath for venilation.
You need to put hay on pallets or it will sweat on an aluminium floor, which will also contain any water getting in, so park it on a slat, so any water getting in can run out, but note there is usually a rear lip which wil hold any water and potentially rot the floor, so seal that with mastic so it can run over and out under the door. Chock the wheels so the rain but not the trailer can run away downhill after rocking in the wind (do not put handbrake on or it will lock on). If you have some old plastic bread pallets, pop these underneath to stop stock hiding under the trailer from the weather and getting stuck.
The gap over the rear door is less of an issue than might be expected, but face it to be sheltered most of the time.
It's not a bad idea to have some space in the back behind the bales so you can get it and pull them down without the stock helping you. Have a step ladder there. Also a handy place to hide if the weather is pants when putting stock out - and entertaining with the back up and you audibly inside with food, they will circle the trailer endlessly like Indians in a Western.
If you are going to put bagged feed in there too, pop it in a bin for weatherproofing, and provide some protection against rats although we didn't have an issue with those or birds, nor need to put rat blocks down.
Even with pallets on the floor under the hay, you will get spoil encrusted on it so after clearing it out, it needs scraping and jet washing, and left to dry thoroughly.
Think I had about 42 small bales in there for about 4 months.
I've also done similar with a six stringer big bale on pallets under a tarp on an old plant trailer. Without a roof there was no condensation but you do need a good tarpaulin and the means to weight it down tightly as you take wads off, as well as moveable pallet end supports to stop the bale expanding aafter the strings are cut and it gets shorter taken off. This was on an old trailer again on which the floor is going to be replaced, and it hammered it. On a newer trailer you'd want it all covered. Was there for about two weeks at a time and a lot less hassle than small bales, as well as cheaper. Tried round bales kept on there but they were a no-go, as if stored on end they seem to distort and grab the floor, making it a pain to slide them out. If on the side they go flat and are hard to roll off. Might as well roll them off when you arrive (assuming you are not on a hill, you won't want to try and stop a round taking out everything downhill!). Same issues as before with wiring, siting, ground etc.