I have certainly heard of Smart Water. But I have not heard of it being used for caravan and camping equipment. I must stress that does not mean it hasn't been used successfully for that purpose.
It can only be a deterrent if the whoever attempts to remove property thinks its been marked, so you would need to have clear signs posted to advise the ner-do-wells. of its use in your caravan.
Otherwise it can help to identify and repatriate stolen property, but its another question, would you want stolen and potentially damaged property back?
I presume the insurance company were about to offer a discount on the policy premium if you have used smartwater, but I doubt the premium rebate would cover the cost purchasing and maintaining the use Smartwater
Its your choice.