We don’t have kids but we have a dog that we would never kennel so our circumstances for wanting one is different but I’d imagine a family of four would soon pay for itself
You would have to factor in the cost of using it (pitch fees etc.), running a caravan and using it on holiday isn’t free. Sites that have good amenities tend can be quite pricey, if you want a pitch were you don’t have to fetch your own water etc. (Your on holiday after all!) then it’s more on top of that.
Compared to going to the EU on a plane, it isn’t necessary cheaper, it’s just different and they offer flexibility (e.g. you can take more stuff, dogs etc.). You really have to use them before they come into their own.
Generally speaking caravans (especially second hand) don't depreciate as long as you maintain them properly
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