It's not difficult to find campgrounds that allow pets. Some will not allow certain breeds, but St. Bernards are not one of them. Some will charge extra for a pet period, some will charge extra for more than one pet, and some don't care how many you've got as long as you adhere to their policies. Call ahead and ask about their pet policy. I've never heard of one charging extra for size. We've always traveled with three pets and I check the area in advance for vet clinics and also let our home vet know we'll be on the road in case he is called for information should one of ours get sick.