Kieth,
Not sure what your wheel well construction is like. I had several problems on my coach with the tire rubbing the wheel well material, as well as the butt welds on the supporting metal straps failing at the top, while on the road. The front side of the front wheel wells gets the most buffeting and that is where mine failed. The photos may give you an idea of how you might handle it.
I made a vertical brace out of the same size material as the original horizontal rectangular tube at the top of the wheel well. 1/8th straps of steel are welded to that, and are drilled for a bolt that will end up above the wheel well rectangular tube to clamp this assembly. These reinforcement braces can be installed without any dis-assembly, even on the road, and requires no welding, so no ECU disconnecting. What is nice about this assembly, is that if you place an extra length of the metal tube in between the welded straps, you can bend the straps to custom fit the angle you want the fender well to follow to give proper tire clearances. Different wheel well locations may have different horizontal rectangular or square tubes in the original wheel well chassis framing. If I was bracing an original strap steel brace, I had a piece of angle steel welded to my brace to fit over it....some braces may be centered in the wheel well, for both support and to set clearance. Your choice may vary, but you will get the idea from the photos.
Whatever Magnum used for the horizontal support, I used that same tube size for the vertical new brace. You may be able to build one and screw a piece of fender well material to it and add the brace with the material to cover the hole. When you bolt the old fender well to the brace it should clamp it all together and also provide proper tire clearance. Though not perfect cosmetically it may be good enough. You could add some black rivets between the old and new fender well patch also.
I did all my wheel wells with these extra braces and have not had a weld break since. Hope this helps.
Later Ed