Re: defense shotgun?
Guns of any kind are never legal in any of the national parks and not in many of our state parks. If a ranger should see you with a gun in any national park, they will write you a citation and impound the weapon. That said, I have at times had one with me, but keep it put away.
As to owning one for protection, although I have owned guns all of my life, I would consider the purchase of one for protection to be foolish for most people. If you do so, make sure that you understand that the protection only comes if you are willing to kill another human being. Give serious thought to that issue before you choose to carry. If you are sure that you will be able to do this then you may be somewhat safer with one if you also take the time to learn to use it well and if you practice with it on a regular basis.
I have probably traveled more miles with a firearm in my RV than I have without, but in all of that time ( more than 35 years) I have never had any reason to use one, or even to get it out. I know many who carry and have done so but ........................ If you do, by far the most safe one to use as well as the most effective is a shotgun. But I suggest that if you choose to do so, you may also want to do as I have chosen to and never say if you have one with you and never display it to anyone. I do own several guns of different types and makes and I might have one, as I often do.
Also realize that you can never take any gun or ammunition into Mexico with your and Canada does allow long guns for sporting purposes, with prior permission but never a hand gun. So do not carry any if you plan to cross a US border. You are highly unlikely to ever have any reason to need a gun, but if you should choose to get one, at least do it right. Learn how to use it well and realize that the purpose of it is to kill other human beings, which is not as easy or clear cut as most would have you believe. To carry a gun and then hesitate to use it will increase the danger in the unlikely event you are ever attacked.
Any rifle or pistol if used is likely to penetrate the wall of the RV next to you and you may well kill your neighbors. In addition, it takes far more practice to be proficient with such a weapon than it does for use of a shotgun, which would not endanger your neighbors. And for heaven's sake, never shoot until you can clearly see who it is that you are shooting or you may well kill your loved one!