I don't know, but I think Elvis was average height, he wasn't very thin (except when he was on a diet and working out regularly) and if he would have been loose jointed he would have probably injure himself multiply times while doing his dances on stage.
And as for the other symptoms I think they appeared only after a long period of taking his presripted drugs everyday, which most definitely took a tall on his body and especially his heart and veins. Haven't heard about his bone probs before. And as for his eyes, skin and lung probs, it could be genetics, or caused by a lot of other things in his stressful life.
I'm no doctor and I'm not saying there's no way he had this Marfan Syndrome, I'm only saying that we can't just jump to conclusions, based on some syndroms, which maybe a lot of people can relate to and don't have the disease in question. Doctors probably have to run some quite complicated tests to find out whether a person is suffering from a particular syndrom.