flavionics,
Kudos to you for looking at your weights. What you discovered is not uncommon for the larger C's on the E-450 chassis. Yes, you are overloaded and I think you should see what you can do without to stay within limits. Perhaps if you tow a vehicle, you might be able to stow some stuff in there (assuming you have the towing capacity available). In my opinion, the individual axle ratings are more important than the GVWR. Having "extra capacity" on one axle does not take any weight off the over-loaded axle.
That "80%" rule that you quoted is a usually applied only to towing trailers, not motorhomes. Even at that, it's just a guideline to make sure you have some wiggle room on your towing specs. So don't try to apply it here.
My recommendation is to try to get rid of the weight, if possible. Then look at new tires with a higher load rating. Pay extra attention to rear-axle related maintenance and brakes. Make a habit of weighing your rig at least once per year
as we tend to put more things in than we take out.
Overall, I don't think you're too bad if you can correct the situation. There have been posts on these forums over the years where the rear axle was OVER its GAWR just loaded with fuel and water.
Good Luck,
~Rick