Split the 4ft boat tail into two halves. Put big hinges on your frame's rear corners.
Latch the two together, and the four caster wheels let it roll sideways. Unlatched and the two swing out over 90° until they make a flat face 14ft wide. Each of the three pieces could have a shelf up to 4ft wide that folds down to make a platform.
If you make the boat tail swappable you could have more than one. One could be a 3x5ft half-bath with composting toilet.
This is easy when you don't have to actually build it.
