[aerohead]

The DOT will allow up to five-feet of elongation with an inflatable tail, four-feet, if 'fixed.'
Since there's virtually zero 'overhang' at the back of a trailer van, and since a proper tail would have diminishing aerodynamic 'mass' as it extends rearwards with its reducing cross-section; and since research promotes ample edge radii for all boat-tail surface intersections, for vortex mitigation, yaw moments, even in strong gust, might not be an issue.
If is was deemed prudent to engineer in, a means to 'erase' an inflated 5-footer,in an 'emergency', off-the-shelf technology would offer us a means to automatically 'defeat' the tail, should accelerometers ever experience longitudinal 'excursions' which exceeded some pre-determined threshold, programmed into the controlling electronics. Blow a rupture panel to collapse the envelope.
On a 'rigid' tail, the same sensor/ actuation could 'unlock' your hinge, allowing the tail to 'unload.' You'd want to limit how far the tail could displace, side-to-side though, as if you happen to be near another rig at that moment, you'd have very little clearance in between.
I'm already seeing 'Trailer Tails' torn up, probably from 'jerk' experienced from repeatedly hitting potholes and such.
Light-weighting the construction as much as possible would minimize the inertia responsible for 'spontaneous disassembly'.
I'd first want to verify the threat posed by crosswind and gust.