I rebuilt a camper once. For lumber I used heart redwood, figured it wouldn't rot if it got wet. All corners were 4x4 with a 2x2 notch cut along the length, so it was very strong on every corner. I used my table saw to rip them. Be careful if you use deck screws, they are not designed to flex and can snap when tension is placed on them from flex. Instead use house grade screws. Lots of types that include special tips to keep wood from splitting and reverse threads at the head to pull in tight and prevent unscrewing. Research on screws will go a long ways to making the camper stay together and yet flex enough when needed. I think most campers are built with long staples for just that reason, but since they are so thin, they rust out very quickly if there's moisture. Redwood also is insect, mold, mildew and fire resistant.