If your tank is permanently mounted it means it's an ASME tank. They are very strong tanks. Some surface rust isn't going to affect the use. You need to get under there and thoroughly inspect it for possible serious rust issues. If all looks good you can sand it and repaint it. They do not need recertification but propane fill places are more inclined to refill a tank that looks good. Many of our tanks are that old and work fine. This is a Toyota motorhome site but most of the camper stuff applies to any RV. Please check that all the appliances, refrigerator, furnace, air conditioner all work before you buy. Appliances are very expensive.
Linda S