Scott,
I do agree with you with respect to the calculations needed to determine the available ccc. However, in the case of my specific motorhome, the calculation of the base numbers takes into consideration many different factors - and the 'weakest link' in all of the items that were included happened to be the tires. I found this only after extensive research. The numbers posted by the RV maker on the vehicle identification plate must have a valid basis. Mine happened to be tires!! For others, it may be the brakes, the axle, the chassis or some other element of the thousands of components in a modern motorhome.
'Your mileage may vary!!'