Routine maintenance like cleaning, inspection, and consumables replacement is very easily automated. Breakdowns can easily be prevented with a combination of redundancy and preventative maintenance. Without a crew you can eliminate many systems that are necessary for sustaining a crew's long term presence which leaves a lot fewer failure points and a lot of room for redundant systems. With modular design you don't need an advanced robot that can fix an arbitrary problem, you just rip out whatever module contains the problem and replace it. It's unlikely on any given deployment that you'd run into a particular problem that can't be handled by an automated system and must be addressed prior to the next return to base, but if you did then telepresence robots, or a team flown over from a nearby ship in the battlegroup would likely be sufficient. If your ship is having a problem that is likely to cause the loss of the ship and a team of experts alone is not enough to fix it, do you really want to have more bodies on that ship?