12,500 is heavy for 15 inch tires. Could be 9,000 lbs on the trailer tires. Check if you can buy 16 inch rims to fit the axles (check that you have clearance). If you can you could put 225/75R16 load range E (29. 4 inch diameter) you get a rating of 2680 lbs per tire. I would recommend Michelin XPS Ribs, fifth wheel trailers eat tires.
If you have the room, you can go to larger tires and get 3042 lbs per tire.