With the sort of resources the SLDF had, I would assume it's the sort of thing that would just sort of be in the background of yard refit, overhauls, major R&M, etc. to customize/configure the gear in the cargo bays for/between missions. So for one set of missions, the Texas class flagship is carrying mostly extra food, parts for all the other warships, dropship and fighter spares, space for 500 prisoners, 4 Titan dropships, 2 mules for shuttling stuff around and a portable HPG because they are going on an extended deep space attack and patrol mission for the next 2 years.
When they finish up that mission The ship undergoes a routine 2 month R&M cycle. Towards the end of it, the invasion of some nameless periphery scumhole is going poorly, so the cargo quarters are configured for an MP infantry regiment for pacification duty, spares and equipment for the regiment, spares and equipment for a battlemech regiment (for when they don't listen to the MP's), 10 extra MASH hospitals with surgeons, nurses, support staff and engineers to set them all up (the major casualties were supposed to be over by now), the 1,000 extra tanks they are going to ferry out to a space station on their way to the battle zone, which they will support with a battlemech regiment in overlords (which they will pick up at the space station) , 2 titans for close air defense, and a single mule for misc. cargo needs. After that they are unexpectedly tasked with assisting the evacuation of the planet Viatran after a massive volcanic eruption poisoned it's atmosphere, and the entire cargo area is converted into maximum density refugee housing to take on as many people as possible for a single jump to the next planet over where all the people are dropped off, and the cycle repeated. The ship then goes in for reconfiguring for the next mission.
If this sounds implausible, remember that this is ~ 300,000 tons of cargo capacity. If 10,000 tons of it is set aside for spare/reconfiguration equipment to make changes to the cargo bay 'as needed', that's a mere 3% of it's total capacity, but represents an enormous capacity to make changes on the fly - ex - if a mere 1,000 tons of that represents the ability to construct infantry bays in the cargo space, you can toss an extra 2,800 people in there with no more than the blink of an eye, no yard, or assistance required - bring it with you 'just in case' Or if you're pretty confident you're going to have a bunch of extra people around for the whole mission, put them in quarters for 5 tons a person and you can bring an extra 1,000 people for only 5,000 tons (1.6% of your cargo capacity) in case you need it.
