Standard large suspension bolts. Similar to those used to hold the rest of the vehicle together...
I don't know the grade but they can hold the vehicle. That's their purpose.
Seriously an air shock prevents sag. It doesn't do anything more than the regular shock does, except compress less because it has air in it.
You can't tow more with them. Mndriver is overloaded no matter what. But if you put 700lbs of tongue weight as 10% of your truck's 7000lb trailer capacity, it'll keep that 700lbs level. That's all. It won't give your 7000lb rated vehicle the ability to suddenly pull 10000lbs. That isn't what they're for. Heck that isn't even what air bags are for.