Mark, I am sorry you had to witness the behaviour of an owner who who obviously does not care too much about his dog, he should never have lifted the animal by the collar and dragged it on its back legs, disgusting behaviour. Dogs shoul dNOT be left alone on site at any time, and most sites actually have rules about this and expressly forbid it. It can get very hot in a caravan during the day, and even awnings can get too hot for dogs, plus , if they get distressed, they can doenormous damage to the van, themselves and if they get free, can cause lots of problems. Personally, if I see a dog left on its own I will report it to the RSPCA and the site owner.
As for getting a dog yourself, and taking account of your son, you will have to do a lot of research on the breeds of dogs, and get one with a very laid back temprament, probably NOT a terrier breed, definately NOT a GSD and not a working breed, such as a Border Collie.
From personal experience a Bullmastiff, Mastiff, Great Dane or, on a smaller scale, Lurcher, are all breeds which seem to know when something is not quite right, and they are very placid and seem to respond to people with disabilities, being much more tolerant to rougher handling by children. However, it would be up to you to ensure your son is taught to treat the dog with love and affection ,and that they need gentle handling and not to be pulled by the ears, etc.
Regarding visiting places where dogs are not allowed, then you have a problem, if it is a trip out from home, then after the dog has settled in its new home, and you are certain it will not cause damage if left for a while, then make sure you give it a good walk before you go out, and most times it wil sleep for the majority of time you are away, but if you are in your caravan, then you have to make hard choices, one of you will have to stay with the dog , its as easy as that, not at the caravan, but at the place you visit, and the other will have to take the children to se the attraction, or, you may find a local kennel which has day boarding facilities.
Sleeping for the dog is easy, in our case, they sleep inside the van, on the floor, sometimes in the awning and they have never caused any problem, except for tripping us up should we have to get out of bed in the dark for any reason,lol.
Getting a dog is a great commitment, but can have great rewards as well, especially for your son.
May I also suggest that you contact the breed rescues for a suitable dog, before paying out silly money for a puppy from a breeding establishment. there are rescue organisations for most of the breeds, search the net, you will find them, or contact the Kennel Club to get the web addresses and postal addresses (they have a web site too )
There are hundreds of dogs desperate for a new home, having been abandoned or just simply unwanted any more.