I was told that the AA will recover your trailer if the tow vehicle has broken down, but if its the trailer thats broken (i.e. the bearings go), then they won't recover you.
I was offerred cover under one of their commercial policies, but that was quite expensive. Could just have been a sales tactic etc.
Decided to buy a load of spare bearings, a better jack, big hammer etc etc. Also joined the carvan club!! as they have an arrangement with the RAC which does recover broken down carvans and "trailers" any type not just just the trailer tent type, but who knows whether they will take a boat (will keep a tent in it just in case!). For £37 I thought it was worth a shot.