Suggestions:
First, stop blaming other people/dogs for your dog's bad behavior. Your dog is YOUR responsibility and if he's behaving badly, that's not because of another dog, it's because you haven't done the work you need to do. When you indulge in complaining about "yappy" dogs and so on, you put energy out that is A) hostile to those other dogs and increases your dog's aggression toward them and B) is weak and therefore causes your dog to continue to feel that he can or must ignore what you say. If you want to change this behavior, make this change... Hereafter, all former "yappy little dogs" will be referred to as "training assistants" and their approach will be referred to as "training opportunities."
Secondly, you need to get your terms straight. A dog rushing to the end of its tether to attack a dog passing by is NOT in "guard dog" mode. He's not in "protection mode." He is in possessive, aggressive, out of control mode. There's a difference. One of the most common things that one hears from people with dogs that have aggression issues is the improper use of the term "protective." A dog that attacks someone attacking you or breaking into your home is "protective." A dog that ignores you and tries to attack something that is not a threat is just possessive, and aggressive.
Thirdly, regardless of whether or not you think other dogs are around, never, ever, EVER tie this dog out again unless A) have done the homework and consulted with a trainer and/or behaviorist, B) you have a solid plan for what you're going to do when barking dogs approach, that does NOT rely on a cable or a collar not breaking and C) you are doing nothing else, not ONE THING ELSE besides controlling the situation with your dog and working on his behavior.
Each time you tie this dog out, you put yourself, your dog, passersby, the campground personnel/owner/nice manager who loves your dog, and every other person who travels with a dog at risk. It is beyond irresponsible. Cables, collars, and things you attach them to break. Period, paragraph. Stop and think for a minute how you'd actually feel facing the owners, their children, law enforcement, a judge, the owner of the park, or any other civilized human being after your dog that you knew was "absolutely in the red zone" on a in an RV park under these circumstances, actually killed someone's little dog in front of their eyes. If you're not sure if that matters, try this. Sit down and write a letter of apology to the person who watched your dog kill their dog. Try to "explain" what happened. Write another letter to all the people whose dogs will not be allowed to visit this park because of what your dog did. Then decide if there's a time when you're that sure that no one is going to need to walk their dog past your site. It's an rv park, not a prison. No one is locked in their rig, nor should they need to be. People walk their dogs at various times and for various reasons. Dogs get out. Yours has an aggression issue. Do not create an opportunity for him to cause another dog harm.
Until you resolve his behavior problem, you keep him on a leash and attached to you while he is out of the rig. That's all.
Next... as others have said, a dog this size that is in the red zone is not a DIY project. If a shock collar won't deter him, a tazer won't either. Trying to physically restrain or correct him yourself is likely to result in aggression being redirected to you. This will leave you with an injury and him with a bite history to follow him all his days.
There are, however, things you can do away from and at the RV park that should help you address the problem. Obedience training is important and once trained you should regularly practice with the dog to remind him of who gives the orders. Nothing In Life Is Free is a non-confrontational way to reinforce your position as the pack leader in his life. Google NILIF. Don't expect to enjoy it but it does work.
You can also teach him a "long down." The way I teach that is this: Take the dog out for a long walk. Bring him back and feed him. Pick a spot in the house where he is comfortable and safe but would not normally rest. Pick a spot that you can ID exactly when he moves away. Put him on a down and tell him stay, and walk away. When he gets up, go and take him by the collar and lead him back then spin him into a down and say "stay" as you walk away. No conversation, no second "down," no other interaction. Put him exactly back where he was.
Keep doing this until he stops getting up and eventually goes to sleep. This is where that walk you gave him will work for you. It's likely to take a couple of hours the first time. It's worth it.
When he falls asleep go to him and release him.
Repeat the next day or the day after. Pick a different spot so he learns that stay is wherever he's told to stay. It will be easier the next time. After he's fallen asleep a couple of times, pat yourself on the back... You have completed the hardest part of the "off switch installation" for your dog. Keep repeating this process in various places and at various times. Always wait until he is calm to release him, but he doesn't always need to be asleep. Once he gets solid on this, it is a very useful command and it also allows you to work pack structure into your day. You can have him on a downstay nearby in the evening and be reinforcing your leadership while enjoying his company. I use this for my hose obsessed pitbull when I want to water the yard.
Also, when you take him to the rv park and have him outside on leash attached to you, you should be working on obedience with him for several minutes a few times a day. Take him to various parts of the site and other areas of the park. He needs to know that you are in charge at the rv park, wherever he is.
If you don't have the skills for obedience work, you need to get with a trainer and get some. This is a large, powerful dog and and you need to know what you're doing in training him.
All of those things are separate from actually addressing his behavior problem when tied out. They will help him respect you more, and make it easier to address that problem with him. But given the intensity of his behavior in the past, you need guidance and help to resolve it safely. For that, you should have him assessed by a trainer or behaviorist and decide on a apecific plan of action based on their observations of him, his disposition over all, and your abilities.
Your best bet may be to meet with the trainer at the park. You can easily recreate this situation if needed by asking some of the owners of the "training assistants" to walk by while you're at the rv park. By knowing when they're coming, you will be more successful in executing your plan, whatever it is. Just be sure that you are prepared to keep them safe.
If you don't have the resources to get a trainer or behaviorist involved or can't find someone you are able to work with, it's simple.... he is out on leash attached to you, only, when at the rv park or at home or anywhere else.
susan
Fuzzy Wuzzy was a wabbit, Fuzzy Wuzzy had a dandelion habit! RIP little Wuz... don't go far.