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.