Very carefully. A tap is hard, brittle steel. Even a large one will snap off if twisted too hard. Use any kind of oil on the threads and be sure the tap is run in perfectly straight. Twist it in a quarter turn or so, back out, clean, oil, twist in another quarter to half a turn, repeat, repeat, repeat. If it jams back it out very carefully, don't break it off. Don't muscle it. Be sure you have the same size tap as the threaded hole are.
M = metric
12 = 12 mm diameter
1.25 = 1.25 mm distance between threads
30mm = the length of the threaded part.
So--does the M12-1.25 bolt look right for the holes? Use a flashlight and, if needed, a mirror to look inside them. I'd think that the bolts would start for a thread or two before jamming on the rust & dirt.
I called the trailer place and they confirmed that m12 1.25 is right. I took a bolt to Home Depot and it was right. I’ll try tapping it. I can’t find a thread chaser in that size anywhere. Thanks for the advice