I am studying a song in Fm and the song has a the C7 V chord. I learned that you write the roman numeral V for this. Then later in the song it uses the Eb7 chord which is also the V chord but in the key of Ab. So do you also use the roman numeral V for this?
Two basic ways to do it: show a key change, or show a secondary function...
Fm: V7 | i Ab: V7 | I ...that shows a key change the two
V7 are specific to the keys as labeled.
Fm: V7 | i V7/iii | iii ...that shows the
V7 relative to
iii using a slash. A key change has not been labeled so it's called a secondary function.
Indeed, C7 is the V chord for Fm. And E♭7 is the V chord for A♭. In the same piece, which hasn't modulated or changed key, V can't be both.
If the piece has modulated or changed key, then it could be construed that A♭ is the new I, which then makes E♭7 V (V7 exactly).
In key F minor, Fm is known as 'i', but A♭ will be III, and E♭ VII.
Could it be that a new section of the piece has changed the tonic from F to A♭? If so, that's why E♭ is called V. Otherwise, to answer your direct question, E♭7 will be ♭VII7.
EDIT: now we have the full sequence, I'd put A♭ as I, and Fm as vi, as in key A♭ major. Making the C7 III7, as opposed to V/vi, which it would be if only it was followed by Fm.