[aerohead]

Okay, so we're limiting the discussion to either tongue-pull, or fifth-wheel, dual-axle RV travel trailers?
Since I only do computer twice a week, I'll throw out some general comments, in hope that they're germane to j-c-c's crosswind concerns:
1) Since there exists no commercially-available boat-tail technology available for 'any' type of RV in the marketplace, I'm still perplexed a bit about the issue of crosswind effects associated with 'rigid' boat-tails.
2) Commercially-produced, single-axle, boat-tailed RV trailers like the Teller-Bowlus 'Papoose' and 'Road-Chief' are reported to be extremely stable platforms when towed.
3) The up-coming, LightShip L1, with an 81-inch boat-tail, has been thoroughly aerodynamically engineered. We can await third-party test reports, as those with access to this trailer make observations available.
4) Anyone interested in adding a boat-tail to any trailer would be highly encouraged to 'clean-up' the trailers forebody, with the addition of some sort of gap-filler, as seen with 18-wheelers, BamZipPow's single-wheeler, some of my monstrosities, and now, LightShip 1L's example.
5) The addition of this forwards 'aerodynamic mass' would compensate for the aerodynamic mass added by the following boat-tail.
6) A boat-tail can only be effective if the onset flow coming to it is already turbulent-free.
7) It follows that, any potential aerodynamic yaw-moment induced at the 'rear' by the tail, would be offset by an opposing induced yaw-moment above the tongue on a tongue-pull trailer.
8) Fifth-wheel trailers attach nearly directly over the rear axle of the tow vehicle. Other than a roll-moment, which 'can' tip a rig over, because of their sharp upper side-to-roof intersections, I'm unsure whether or not forward yaw effects are even experienced. Around 15% of the trailer weight is added to the static weight already acting downwards on the rear tires.
9) I've pulled fully-boat-tailed trailers equivalent to coast-to-coast across the USA, experiencing up to gale-force crosswind in the Texas Panhandle, Phoenix, Arizona haboob wind storms, and been inside large dust devils near Tonopah, Nevada without event.
I'll await further information.