You are overweight on your rear tires. The question is are the tires under rated for the chassis or are you exceeding the weight limits of the chassis?
What's your GRAW rating on your door sticker? With those tires it should be around 16,000 or 17,000. If it is then your exceeding the weight rating of the whole rear axle, not just the tires.
I don't know what that axle is really rated for by it's manufacturer. It may be that the GRAW is based on the tires they installed on the motor home. You could install higher rated tires and be fine, or maybe not, depending on the actual weight carrying abilities of the axle assembly.