With the arrival of a 30-7934 operating dump car cataloged in the 2000 volume 2 I am definitively able to answer my question. This car just came from MTH's own stock, apparently the last one from 15-years ago.
This is the image from the MTH website for this car. Which is the same as the catalog images from 1999 and 2000 for RK operating dump cars. These catalogs state length as approximately 10 1/2". Also the same image for the RK dump cars pictured in "A Toy Train Story."
This is a pic of my car.
So it appears that the cars RK and Premier were in fact all made the same although originally cataloged and picture in the MTH book differently. So either MTH intended to make two different cars or it was all "artwork" variations.
But one big difference, the price. My car MSRP was $39.95. Paid less than $35 new from MTH stock.
I knew there was a reason for thinking that there were different cars.
Ron