Well Jono, I have been known to dunk and retrieve our 6.3 OP from Lamlash slip using the Transit SWB - no problems so far even on our shallow but Teflon-coated slip, so if your mobile roadblock has more weight over the axles than an empty van you should be OK.
Then again, if a camper van was dragged backwards into the oggin I don't think it'd be any great loss.
"Not that I am biased..."