Steve,
If your tires are overloaded, they need to be replaced with tires that are adequate for your coach's weight.
When I purchased my 2000 Marquis, it had new Toyo tires that were identical to the OEM tires. When I weighted the coach, I discovered that the tires were overloaded. I went to Les Schwab and traded in my tires (very reasonable exchange) on tires that had a sufficient weight rating.
There are several things that you need to keep in mind when you change tires. First if you change to a larger size, make sure that you have enough clearance for the tire, especially when turning. I spent hours with a tire dimension spec. book and a tape measure to be sure that the larger tires that I wanted would have enough clearance.
Second make sure that you do not have a width problem. Your wheels have to be wide enough for the new tires, and if you change the rear tires, there has to be enough offset to the wheels to allow clearance between the tires. The tire manufacturer's tire spec. book will have all of the tire measurements as well as required wheel width and minimum dual spacing.
Gerald