this does require slightly different analysis methods when designing the structure, especially for fatigue, which will be critical for this application.
From my real world experience with a camper frame (made with 6061 1.5" square tubing and skinned inner (4mm plywood) and exterior (Al siding)) bolted down to a steel 1.5" thick flat deck with similar 12" rearward extension from the rearest frame mount, it is the global ensemble of the camper & deck assembly that offers the "bending" strength. In another word, the cross-section of the complete assembly is actually the full height of yr camper, and not just the thickness (in height) of the 12" frame extension you are trying to add. Hope this makes sense...
this does require slightly different analysis methods when designing the structure, especially for fatigue, which will be critical for this application.
