Removed hitch - holes in bed of truck

You can go to your local Fastenal store and purchase Stainless Carriage bolts, nuts, washers and lock washers. Before you put the washers and nuts on the bottom side, squeeze some adhesive caulk around the bolt threads. Then tighten the nuts down.
You could also put a little caulk under the carriage bolt flange to seal it to the bed liner.

Bandaid

just leave the carriage bolts in the holes, with washers on the bottom.

I closed off each hole from underneath with a piece of Gorilla Tape. Then up top I dropped a penny in each hole and then filled each hole with J B Weld. Smoothed the J B while it was pliable and let it dry overnight. I then rough sanded the J B to conform with the bed floor and sprayed each area with rubberized undercoating spray. Turned out so sweet that you can't even see where the holes were on the sprayed in bed liner.

Cheers!

Mike

Wire brush the underside around the holes, then inside of the holes, and then simply paint the area with oil-based Rustolium. Find some kind of plug that matches the hole size and seal it in with a good calking.