I've been good, thanks! Just been busy with work, that's all. How have you been keeping?
Concerning the structure, I'd probably get some inspiration from the Morgan body structure. I think what they do is that they have a steel backbone onto which a wooden structure is attached to take the aluminium panels.
About the differential, you don't need it since you won't be driving the wheels. What you do need is a beam axle of the type found under the rear of a front wheel drive van - for example a Mercedes Panther. You could use a differential, but the problem is the fat middle section which would necessitate a high floor in the caravan.
p.s. There are no silly questions!