Original RGS cars were mostly gone by 1903 (not worth converting to automatic air & couplers) a few that had been added/rebuilt remained until WWI era. By the 1920's only one lonely flat car was lefet.
The Miller cars came in 1938 but were actually the property of Victor Miller. After the dust up which resulted in Mr Miller leaving, there were several years of negotiation on their status with most of them being sold to the Chicago Freight Car Parts Co. in 1942. 40 or so stock cars, a reefer or two and, I think, a few boxes in work service, were all that remained to the end.
RGS between 1895 and 1929 was considered part of the D&RG(W) system (but not included in the big 1908 merger of all other D&RG subsiduarys) and used D&RG(W) freight (and quite a few passsenger) cars, for which the Grande charged the RGS an (exorbitant, according to some) fee, just like with engines used.
At some point during the 2nd recievership (1929-1953), I think around 1940, a deal was struck where the RGS and the Grande quit paying for the use of each others cars.
hank
PS I'd like to point out again that the RGS was profitable in the period 1896-1912, in spite of the Grande bleeding it dry. I haven't dived into the deeps of the RGS accounts but I have heard that the RGS would have been profitable from 1914-1929 if not for various questionable charges that D&RG(W) laid on it.