I've been daydreaming about this.
Generally the SS takes the lead on presidential security, but they don't do it alone. In DC they coordinate with local police and the National Park Service. When the president travels, they coordinate with local police forces as well.
So if Trump went to jail, I could see the SS working with the regular prison guards. They could screen guards assigned to Trump's area, oversee (and even fund) improvements to the facility to help it hold such a prisoner.
Like I am imagining a single SS person on staff at all times, but not necessarily right there with Trump. Maybe someone in the Prison's surveillance/command center, who also a few times per shift does a walk around. But 95% of the security would still just be prison guards, albeit well screened and hand-selected ones. This assumes that Trump is segregated from all other prisoners. Maybe he gets a cute little trailer in it's own little sub-compound off in the corner of some larger prison complex, or even in a walled off area of a law enforcement facility.