As someone who tows both a trailer, and a caravan, I personaly get my caravan checked bi-annualy, ie: gas/electric/water checked one year, and wheels/brakes/coupling/suspension the next, on a rolling basis.
The caravan gets frequent use from March through to October, before being 'laid up' on the forecourt, using 'winter wheel stands' and the wheels/tyres stored inside out of the way.
The trailer gets a lot more use, is only 5' x 3', but gets an annual 'check over' by myself, and a check over EVERY time I use it.
Having towed various caravans for over fifteen years, with a variety of cars, I have yet to have an accident. Most of the accidents appear to be due to incorrect loading, or innapropriate combinations of van/car.
Having a wife and children to consider, it costs so little to have checks done, in relation to the cost of an accident.
I pay £50 for a mechanical check, (brakes,suspension,coupling,chassis) and £60 for 'habitation' (gas, leccy etc) plus the cost of any parts, which to me, is a small price to pay for peace of mind.
Not sure if a MOT if the right way to go, but some form of paperwork to show that reasonable checks/repairs have been carried out/done seems a sensible way to go.
BTW, I informed my insurance company that I towed both a trailer, and a caravan, and they didn't seem bothered, but it may pay to let them know just in case..