I looked into a trailer tent having seen some at our local camping shop, there is a really handy section on ukcampsite which has loads of info in the old posts.
Came to the conclusion that if it was to be a trailer tent then it would have to be a Camplet or Combicamp or one of the quick to put up Raclet ones, as some take a fair time to put up, which defeated the purpose of why I wanted one.
I then read a bit about folding campers and we decided to go down that route instead and are now the proud owners of a strange looking and sounding thing called a Dandy. It is made from PVC so doesn't rot and can be folded down wet (though in reality you give it aquick wipe down with a towel when folding it away). The roof and the walls are insulated so it is warmer when cold and cooler when hot, though only marginally I suspect. Not everyone's cup of tea but I love it.
Ours is similar to this one but doesn't have the end window. I got some cheap fabric for £20 and have recovered all the cushions, taken out the carpet and replaced with lino and made new curtains, and hanging pockets for storage, it now looks fab and the children love it. Has electric hookup and leisure battery fixings (we haven't actually got a leisure battery atm and don't intend to replace it).
Takes 10 mins to put up when you know what you are doing (Have put it up and down a few times with DH and haven't as yet had an argument)and we have a Khyam awning that pops up in 5 mins (plus pegging)and attaches to the awning rail, if you want to put it up. No pegging out of the main unit. Porta Potti lives under the Dandy by day and in the tunnel between awning and Dandy at night. We haven't had it long so only have been away once but loved it and can't wait to go again. Cost £790 plus the bits I spent on renovating it plus the awning, but lots of other cheaper ones you can get.
Was very worried about towing but it is a doddle and doesn't have much impact on the fuel economy. Other makes of Folding Camper are Conway & Pennine.