I have spent ages (ok, half an hour) looking for the best deal for breakdown cover that would include a caravan if we were to break down while towing, and it's quite difficult to find the info. So I rang The AA, and lo and behold, they will include a caravan for no extra charge on any policy. There are limitations - max 3.5tonne, 2.4m width, but I think the majority of vans should be fine with that. So whatever policy you've got, your caravan is covered to the same limit. You don't need to tell them, it's just automatically covered if you're towing it while you break down.
Basic cover from £27 online, less £17 cashback = £10. This will tow car and caravan to the nearest garage if it can't be fixed at roadside.
But for £20 extra, take out the Relay option for £77 and get £40 through Topcashback/Quidco, so it's only costing £37 for full cover - that's the car & caravan to any UK destination.
Even for those who (for some unknown reason) don't use cashback sites, it's helpful to know that you don't have to pay extra for caravan cover.
Fully expecting lots of cold votes, as caravans just aren't cool enough for the majority of you dudes...