Unearthed Arcana: Revised Class Options shows the Eldritch Invocation named Kiss of Mephistopheles:
Prerequisite: 5th level, eldritch blast cantrip
When you hit a creature with your eldritch blast, you can cast fireball as a bonus action using a warlock spell slot. The spell must be centered on the creature you hit with eldritch blast.
My warlock is level 12, meaning all her spells slots are of 5th level.
By RAW, would the triggered fireball in this case also be cast at 5th level?