I have a huge (for me) leg of pork that's 5.9kg/13lb. I'm thawing in the fridge for the next 4 days before cooking.
How do slow cook for tenderness/fall off the bone as well as great crackling?
Slow cooking and great crackling do not go hand in hand. Your best bet will be to remove the skin, score and salt it well, and grill/broil it separately.
For the leg itself, low and slow is the way to go. How long depends on how long you've got, but 5 or 6 hours in a low oven will do the trick. Start off with the oven on as high as it will go, throw some garlic, onions and herbs in the pan with the pork, add a splash of water, and cover with foil, then put the meat in and lower it to 160C/320F and go read a book.
You want to make sure the meat reaches 70C/160F internally, incidentally.
I roasted a 6 kg leg of boned, rolled pork on low heat for 12 hours (overnight) with the skin on (rubbed with crushed marjoram, fennel seeds, caraway seeds and a smidge of salt). When I removed the pork from the oven, I took the skin off and spread it on a baking tray (I wrapped the pork roast in foil and set it aside). I turned the oven up as high as it would go and put skin in for about 20-25 mins. Perfect crackling, and the pork melted as we pulled it apart - delicious, and I will be doing it again.