Had a gander at the aluminium plant trailer at work as to how the suspension and coupling is attached. The coupling is attached with HT bolts, and the suspension is welded to 150 X 75 X 10 angle, then that is bolted to the aluminium chassis, which is 200 X 75 Channel. The channel is rolled into a curve, similar to the link below.
Aluminium Plant Trailers | Sureweld
The caravan chassis would have outriggers to support the floor and composite sides, as per the image below.Attachment 377240Attachment 377241
Have found an image as to how composite caravans are attached to the framing. Sealant would be used on all the joints.
Attachment 377239
Hope this helps.
Kryn