My parents had a trailer tent and I have very fond memories of all the holidays we had in it. However me and my sister were about 8 and 10 when they got it so didn't need constant supervision.
ds1 will turn 4 next summer, dd will be 2 and ds2 will be nearly a year. If we were to get a trailer tent for next summer I can imagine it taking hours to put up as I try and keep the kids from running off whilst dh has to put up the tent single handed. Also even when the tent is up I imagine that it will still be hard work to keep a constant eye on them and stop them running off in all directions.
Is it worth even considering for next summer? Or is it just not going to be a holiday, just childcare in a less well set up location than home? If so at what sort of age does that lack of a contained area that you would get with a cottage with garden stop being such an issue?