Although a number of semi-independent fiefdoms seem inevitable, I think that the divisions in a post-victorious Reich would be geographical rather than political. There would always be a fuhrer though, a unifying and powerful arbiter of the whole system.
The Gauleiters were always powerful figures, even able to mess around with high SS like Eichmann when it suited them, but the fuhrer could intervene without protest to settle any dispute anywhere.
A nazi superpower based on a system of gaus and protektorates running from the Atlantic to the Volga would be very stable I feel. The nazi doctrine was always based on the fuherprinzip, so a ruling oligarchy can be ruled out I think, though there could easily be a civil war at the death of each fuhrer.
A good model could be that of the Roman Empire post-Augustus, with the gauleiters and higher SS playing the roles of the governors, proconsuls and legates of the Roman system. Maybe a civil war every fifty years or so, then stability for a time until there was another disputed succession. The Army, with its personal vows of loyalty to the fuhrer and the overwatch of the Army by the Sipo and SS, should never be a danger to the Party.
Such a Reich would essentially be the Eurasian World with no real frontiers...Africa and South Asia would be composed of colonies or gaus, and the Americas very distant neighbours, of little threat.
As the Roman Empire went for well over a thousand years, including East Rome, I think the "Thousand Year Reich" was well on the cards. In fact I think that if a modus operandi was worked out with the Japanese, there would be no reason at all why it would ever fall at all, becoming in time a society based on rigid ethnic and cultural lines like the unified China founded 200 BCE.