I installed my hitch about a year ago. Sprayed the hell outta the holes with PB Blaster, used a wire brush that was just barely small enough to fit in. Spun the brush a ton, cleaned it off, sprayed the holes again…and on and on.
Then took one of the old bolts, sprayed it but good with PB and ran it in and out with the impact. Cleaned it off, did it again. And again. I didn't have a tap the right size (but I do now) or I would have used a tap to clean it up.
Once I did that, I could torque the new bolts up tight. So clean up the holes before you take it in and you'll be fine.