I want to go back to Twilight Town to collect the treasure chests, but I'm way past that point and can't go back and I use only one save file that I already overwrote. Is there a way for me to load an older save file after
I believe you already know the answer to that. But don't worry; apart from the ones in Roxas's story (which all contain common items anyway), not a single chest in this game is permanently missable. Just bear with the empty space(s) in the journal and wait for the next opportunity.
the chests you can find as roxas aren't important for 100% completion. nothing required for the journal is missable in game. in fact, you can go back to those areas in twilight town later in the game and find those same chests, and they will contain better items. one chest that formerly contained a potion or something similar now contains an orichalcum+, for example. so don't sweat it.