The fixture you mention is fused at the courtesy light lamp fuse holder.
You mentioned replacing the alternator and battery, with what?
Another used used battery and alt?
No mention of load testing the battery, or electrolyte levels.
Did you test the "New" alternator with a load on it ? ( meaning every
electrical item on in the Van?)
Maybe if the fluorescent lamp had a direct short to ground it would
drain the battery that fast.
I really doubt it as you described the situation.
Buy a new alternator, and battery.
Used isn't always the best.

RC