F416376

I had to purchase a Samson tire in middle of nowhere when one of my Michelin tires had a blow out. I am in process of replacing 5 year old Michelin tires on rear axel with new ones. The question I have is, do I need to replace all 4 or only 3 and keep the Samson but keep it on an outside position? Tire size is 275 R70 22.5 for a 40 foot RV Damon Tuscany. Any thoughts?