Hi, that is one tough question!
Unfortunately, dogs don't tend to understand when they are meant to be protective and when they are not.
It may pay to try a technique with your dog where you teach him to relax. It simply works where you ask your dog to sit and relax and as soon as he shows signs of calming down and relaxing you tell him good boy/or give him a treat. You must wait until he looks relaxed - this may mean that his ears go slightly limp, or at least has his complete focus on you. This works really well in dogs that are anxious because when they are in a stressful situation, you are able to tell them that there is no need to worry - you are in charge.
Start the relaxing technique while you are at home. If he learns this well you can try this at the carvan park - as soon as he starts growling, call him to you and ask him to sit and relax. This should at least distract him for a bit and hopefully he will start to understand that he is not needed to protect you all the time!
Let us know how you get on
It sounds like you have a really lovely dog!
Kate