Before you spend money on a dog crate, I suggest you buy a book on how to look after your dog. This will explain how to introduce him/her to being crated.
3 weeks is just about enough time to get her used to using and feeling safe in her crate although some dogs take longer and some will never accept a crate.
Each of our dogs was caravanning as soon as they had had their final injections and were allowed to mix with other dogs. They have never been crated on a campsite although we did use the crate in the house for a while.
"Toilet accidents" will happen with any puppy and it is something you will need to be aware of and how best to deal with them. Our breed, the Bichon Frise are notorious for being difficult to housetrain and I know of a few that are 3+ years old and have still not got the hang of it. However that IMO that has more to do with owners that were too intelligent to need to read any books on the subject. 
Dogs of any age, love the sights, sounds and smells that they experience on a campsite and ofcourse the new places they visit when touring in a new area. As long as we are aware that things look different when you are only 6 or 8 inches high and that certain other animals, IE Rabbits are not put their exclusively for their entertainment, then I am sure that both you and puppy will have a wonderful trip and she will be much admired by the oher campsite users.
------------- Lobey.
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