If it were me i'd be looking at sealing from the outside, does
water hit the walls when it rains? are the boards sealed at all? do gaps widen and narrow?
guessing you have extremes of humidity and temps
wood can last a long time on a shed wall if it doesn't stay wet, but each time the temperature goes up or down, each time the humidity increases or dry out the boards will move, swell and shrink and even bow and flex depending on how they are fastened.
classical sealing for boards is usually done with battens, thinner strips of wood that cover the joints of the boards, i generally try and put the nail through the batten into the joint between the boards to catch the horizontal members,or nail the batten to one side or the other for the entire length , that way there is less splitting of the original boards and everything can continue moving but still have a more weathertight joint,
heavier wood (log cabins) are often chinked with mud, but they are not prone to as much movement, and even then there is an ongoing maintenance