I had thought about one of these myself, being a newbie to caravanning.
Take a look around any storage yard, as I have done, this will indicate how few
caravanners use them, possibly one per cent.
I did ask one caravan engineer about the apparent lack of take up on these
and he reckons the risk of scratching paint with any stray bits of muck, together
with the alleged lack of breathability of some designs, leads to problems you could
do without.
I'm going to hang fire on getting one, until I'm 100% sure of their efficacy.