It sounds like you have a good handle on living with a dog in a vehicle, actually. I have to disagree with mayble that one of the highly cold-sensitive breeds would be appropriate for you, though, even if you do intend to almost always keep it with you. Between the fact you want to spend a lot of time outside in colder weather with the dog (I assume) and that the van may get pretty chilly in winter at times, you'd be best off with something like a lab which will definitely not be suffering sooner than you are. Many of our farm dogs while I was growing up were labs and they definitely handle cold very well without having the heat sensitivity huskies do. Labs are very high-energy and slow to mentally mature, though, and aren't going to have much ability to be trained for guarding.
I could see a rottweiler being a match to almost everything you want except that they certainly will scare some people. My experience is rottweilers are generally really sweet and cuddly, but it's in their nature to protect/defend their home and family if you're threatened. You wouldn't probably need to particularly train them to do that either, and I don't think they're really cold sensitive. They are on some insurance companies' banned/dangerous dog lists, though, which can cause problems if you wanted to do something like stay in a campground or pet friendly motel. They're great dogs usually, though.
Any breed ending in 'mastiff' is also going to naturally guard you, and they tend to be low-energy too, while still being quite capable of hiking and pretty tough physically. They're usually very cuddly and affectionate with their owners too. These are NOT on the banned/dangerous lists (possibly because they're just less popular), so you wouldn't have problems with that. They're huge, though, obviously.
If you ignore the guarding part, golden retrievers and springer spaniels are two common breeds which would perfectly meet your other criteria. Springers are usually a great size, as they're smaller than most of the other breeds I'm listing but still plenty large enough to safely enjoy the outdoors and hike and camp.
Another huge thing to consider is whether you want to get an adult rescue dog or buy a puppy from a decent breeder. There are big advantages and disadvantages to both, honestly. Obviously you get to skip the puppy stage, save money, and get the satisfaction of giving an unwanted dog a home with a rescue, but you have no idea what their previous experiences have been, so may discover they have neurotic behaviors or certain phobias from their previous lives which could become a massive problem in a vehicle-living situation (imagine if they were phobic of riding in cars or of small spaces). They also have unknown breeding and medical history, so could have more expensive health issues younger, especially with a purebred shelter dog who quite likely originated in a puppy mill. A puppy from a decent breeder will cost more, but will have parents who were health and behavior tested, often going back multiple generations. A puppy will also very easily adapt to your lifestyle and won't have weird phobias from their past. You'll be able to raise and train them just how you want from the very beginning. Of course, the biggest downside other than cost is that you then have to deal with raising a puppy, lol, complete with all the middle-of-the-night potty trips and chewing phases. You'll have to housebreak them, teach basic commands, and wait until they're old enough to safely hike and camp. Going to a breeder vs. shelter is a really personal choice and there's no one 'right way' to get a dog.
