Your in luck.
The ST225/75-15 C load range has a 2150 lb load capacity.
Goodyear Wrangler HT in a LT215/75-15 D at 2095 would be a good upgrade from a ST tire. 2095 lbs times four = 8380 lbs of capacity.
Vanco 2 from Continental another popular tire for trailer users has several 15" sizes and load capacities. 215/70r15 at 2271 lbs capacity @ 65 psi. And a 2250/70r15 at 2470lbs capacity at 65 psi.
Do your home work on wheel well clearances and tire/wheel load and pressure ratings.
"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers
'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides