Your question about which would be best, a Mini or a Standard Schnauzer depends on why exactly you are getting the dog.
The first thing is that you are planning on living in a rv. In this case, in a confined space like an rv, I would strongly recommend getting a smaller dog, like a Mini Schnauzer.
Also you will be spending time indoors in a small space with your dog. Schnauzers are people-oriented dogs, so this would not be a problem for the dog.
All Schnauzers will bark if someone comes to the door, so they are good watchdogs, but they are not known as "attack dogs".
Most "attack dogs" are larger dogs which have been specifically trained for the purpose.
You have also said you will be walking the dog, but there is no guarantee of free space. In this case a smaller dog which needs less exercise would definitely be better.
As for travelling in a car, if you start off when the Schnauzer is a puppy taking the dog for short journeys to get used to the car, it shouldn't be a problem.
A couple of other sites for info are the AKC - American Kennel Club - and also try one of the Schnauzer forums (type "Schnauzer forum" into Google) for more info.
I hope that's helpful to you and good luck with choosing a puppy.
