The issue with OSB is that if and when it gets wet it will swell up and crumble into the sawdust it was made from.
Exterior grade or Marine Grade plywood will not. And the newer Azdel and synthetics will out last the Pyramids!
Also picking an RV made with welded Aluminum tubing and High Pressure Vacuum Laminated walls, roofs and floors such as all Jaycos and Forest River RV's have will also pay off in a long lasting structure. Jayco says that Vacuum lamination is 10 times less likely to leak than others, and Jayco also doubles the sealant in their seams.
Evergreen RV, CampLite a few Dutchmen and a very few others use 100% composite walls, and the price of the unit will reflect that! 30% to 60% higher.