use some telescoping square tube, weld 3 attachment points under the door and 3 more on top of the wheel tub in the trailer so you can store it, then just build a nice step out of square tube and cover it in a piece of diamond plate. Nice, easy and simple.
You could build an aluminum hinged one that folds into step opening that way if you build it strong enough you can drive golf cart over it if you need to. We haul 4 wheeler and golf cart and it is pretty tight not to run in opening.
I have a Pace silver Arrow aluminum trailer. Pace made mine and it is exactly what you are looking for. I thought I had a picture but no luck. The frame is 1" aluminum tube. The step is ATP. It slides in on rails like a C channel. It locks in with pins and out the same. It is basically !" tall so is does not drag. The trailer is in storage. If I get a chance this weekend I will try to get a couple pictures.
The step came from Pace. I think anyone with a welder and some aluminum can make it cheap. I can't remember what the option cost me but I think around $150-175 range. I will get some detailed pictures this weekend. The trailer is in storage about 40 miles from my place.