I have just been through this loop too before I took the boat to France for 2 weeks ! Strange how when I called the AA to cancel my cover so I could get car and trailer cover elsewhere they swore blind they would come out to a broken down trailer - I am still waiting for them to highlight the wording that confirms that and post it to me though ;-)
Anyway - I joined the Caravan Club and then took out their Mayday Cover with Green Flag (more cover than going direct to Green Flag). It explicitly covers the tow car and the caravan or trailer behind it. If is a caravan then they seem to cover some repairs and incidents involving the caravan itself but if a trailer then they only cover the trailer and not what is on it - eg a sterndrive with a jammed tilt etc. However bearings wheels axles brakes hitches etc are all covered.
The best bit is that their cover will also cover onward journeys and allow for fixes with garages etc at your destination so you can carry on to your holiday and still use the boat while your car is being repaired or while the boat trailer is being fixed etc.
The AA cover I had was full Joint Personal Comprehensive and any car and cost £119 I think but the Caravan Club only £6 more (including the cost of being a CC member of £35 !!)
I also then took out the Caravan Club Red Pennant European Travel and Motoring Insurance for the 2 week trip to France to extend that breakdown cover and also add the usual travel insurance cover etc and that was £154 for complete peace of mind. If you trawl the Caravanning forums for comments on the Mayday and Red Pennant service you cannot find a bad word so for me it was worth it.
Also check some of the small print on some of the other operators who seemingly offer trailer breakdown cover as part of the motoring cover - some wording offers to come out and have a look to see if it can be fixed roadside within half an hour but if it can't then charges are on top for recovery etc.
The Red Pennant cover certificate came thorugh explicity stating I was towing a BOAT and not a caravan.
I also had both hubs / bearings and brakes all changed before I went as well but the SBS trailer on the ALKO axle has Sealed for Life bearings and the cost of hub and bearing and dustcap together is over a £100 per side PLUS fitting....I think you can buy a whole axle for not much more !
Hope this helps
(but not what is on the trailer