I would say that unless the caravan is worth quite a bit of money, the cost of having such a repair done would prove quite uneconomical. The materials required are probably not expensive, but the labour costs are horrendous and that would not be a five-minute job.
If you can't or don't want to do it yourself, unless your van is worth a fair bit, it may be cheaper to replace it. I know that the cost of such a repair on our caravan would be considerably more than I paid for it, so if it needed that much work I would either do it myself, with the very useful tips on his site, or get rid of it.
Thing is here that you suspect the boarding below the window, but with damp think iceberg - 10% visible, 90% revealed when the old boarding removed.I know you don't want to hear that, but its nature of damp I'm afraid, and why your question is impossible to answer until the van is stripped out...
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