Either way you will need to do a lot of reinforcing on the frame. If you use air bags it is going to also need a different axle with a lot of drop in order to get the trailer to drop just a few inches. This will also reduce clearance to the road. The axle is going to need to be located front to back and side to side, at least 3 points. The top air bag supports will have to tie into the main rails and nearest cross members. All these areas will have to be well reinforced.
A lift will require a lot of reinforcing to distribute the weight of the lift, chair, and occupant. These will be probably be 2-3 feet from the attaching point. Leverage and the total weight are going to overload what the frame was designed for. The weight should probably be distributed over several feet of the frame rail as opposed to just the width of the lift. Neither is impossible but both will need some careful planning.
Search for axle replacement and you will find some pictures of the Henschen axle and mounting and why it won't work with air bags.
Look at
this thread and
this thread. There are some good pictures which will help you understand how the frame is built.
John
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