I am running a standard parallel 4 link with bag over axle. The largest i've towed is a 2 car enclosed trailer with 1 vehicle and a crap load of car/race parts. I do have double convoluted bags which are used on some Semi trucks, and i've seen them on some large dump trucks and some train cars. 7800LB. Works great with a load, but with no load, its to much and is a bit bouncy. I've got a cab & chassis rear that i'm going to be putting on it soon, and moving the 4 link bars to the inside of the frame. Just got to finish doing the disk brake conversion on it. I'll get some of the construction pics up tonight or tomorrow.
http://www.lowdowncustom.com/gallery/v/phil98dually/