Billinwoodland,
While I'm hesitant to say "all", I will say most 30'-ish E-450 C's with a slide or two will be pushing the weight limits. As you mentioned, it's usually the rear axle weight rating that is nearest it's limit. There should be a sticker somewhere in the rig which states all of the weight ratings and an unloaded weight as well. But it's still tricky. If you really need the space of a 31' foot rig, I'd look at the super-C's, or a class A on a 22,000 lb. GVWR chassis.
If manufacturer's give you an unloaded weight, it's usually for the entire vehicle, NOT the individual axle weights. On the long E-450 rigs, it's common for the front axle to be 700+ lbs. under it's limit. However, since you are really limited on loading the front axle, it's unusable capacity. Yet, the manufacturer will include that 700 lbs. as cargo carrying capacity. Buyer beware.
When I was doing our research, I estimated we'd need between 2,000-3,000 pounds of capacity for water, food, fuel, clothing, people, dogs, tools, supplies, etc. The only way I could get that and stay within the weight limits was with a 22,000 lbs. chassis. Our rig weighs about 18,800 lbs. unloaded, so it does work for us.
Granted, this increased the price of the rigs we were looking at, but in retrospect, it was well worth it. We bought a rig which was always operating within it's design limits and was large enough to accommodate our growing family. Ten years later, we're still very happy with the overall floorplan and amenities. Most folks keep trading up, losing money on each trade due to depreciation. We bought right the first time and have no regrets.
The majority of RV'ers don't even concern themselves with weight issues. They just figure if the manufacturer built those large storage bins in the back, it must be OK to load them up. As you said, the back end of these is usually heavy to begin with, yet that's where all of the storage is, so loading up those bins really stresses the rear suspension, rear axle and rear tires. Then you read all sorts of stories on here about poor handling, blowouts, etc. and they wonder why. I'm too analytical to accept that and I knew operating above the design parameters would both me.
My best advice to anyone looking at these rigs is to take your time, do your homework and make sure that you can find a floorplan that works for you and your family on a chassis that can safely carry the weight.
Oh, and if you'll be carrying passengers, make sure there are sufficient seatbelts installed. Ours had an almost unheard of 12 belts (3 on each couch, 4 at the dinette, driver & passenger) installed by the manufacturer. Not that we use them all, but it gives the kids options as to where they can sit on a long trip. You can always add additional belts yourself, if you want. Most dealers WON'T add them due to liability concerns.
This forum was a HUGE help for us 10+ years ago when we were doing our research. It really helped us zero in on the rig that was perfect for us. There is experience on these forums unlike any other. Yes, you sometimes have to put up with some strong opinions, but that's the internet for you. :)
Good Luck,
~Rick