System control ships
I understand there's also been some discussion about using battleships as system control vessels because of their manpower numbers. That is, if I understand the way the notion was posed to me, that the life-support capability of battleships would make them logical basing platforms for Marine units designed for local peacekeeping and garrison responsibilities while simultaneously providing a sufficiently capable combat platform to dissuade possible outside attacks on the system.
The idea may have some merit, but I doubt that it's going to happen. It would be far more likely that the RMN would assign some of the old, obsolete wallers -- especially any of the dreadnoughts which remain in commission -- to this role rather than constructing new-build battleships (and, of course, they don't really have any old-build battleships). It's unlikely that they would even use dreadnoughts, but at least that would be a case of using/modifying existing assets. A dreadnought is large enough that it could probably sacrifice some of its existing armament to build in additional life-support space for a sizable force of Marines. Even that would be extremely difficult, because of the armoring scheme built into the ships, and the new quarters would probably have to be shoehorned into the existing magazine spaces. One reason it's so difficult to refit existing wallers with things like upgraded missile tubes is that the damned things are built so tough that even taking them apart in shipyards is a major undertaking. This is also a primary reason that the Manticorans didn't really consider simply overhauling and refitting existing ships of the wall with the new automation systems. It was faster and cheaper to build from scratch.
(As an aside, the Solarian League's penchant for refitting existing hulls despite the difficulties is another reason that their ships lag so far behind the technological curve. Electronic warfare systems and sensors which can be fitted into existing electronics spaces are relatively easy -- those were the primary changes the Graysons made in the captured Havenite superdreadnoughts the Royal Manticoran Navy gave them before Flag in Exile -- but changes/upgrades in weapons systems and other components nested deeply in that honeycomb of armored bulkheads and barriers is something else again.)
I would presume (although, of course, I wouldn't know) that if the Manties used a ship for this purpose at all, they would be more likely to use one of their existing assault transport designs or else to build a completely new design, based no doubt on a merchantship hull, to support the Marines and their planetary assault craft. The traditional approach when it comes to maintaining order on a planetary surface has been to establish orbital platforms fitted with kinetic bombardment weapons as well as life-support space, small craft docking facilities, and maintenance space for the local quick reaction force. In any case, such operations are properly the role of the Royal Manticoran Navy Army, not the Marine Corps.