As one poster says look on the back side of the wheel for a decal or stamp and also some wheel ratings are in the valley which means the tire has to be taken off the rim. If your OEM tires are a 15" ST E then you may have a 5 or 6 lug hubs.
F load range should be at 95 psi. You may have to go with a 16" rated at 110 psi which usually are 8 lug wheels.....which requires 8 lug hubs.
The best upgrade from 15" load E ST tire on 5200 lb axles is going to a 16" LT E at 3042 lbs or 2680 lbs capacity in either the Bridgestone R-250 or the higher price Michelin XPS RIBS. Both are a commercial grade all steel ply carcass tire. Both have been recommended by their mfg for commercial trailer or rv trailer use.