I run specific trailer radials . Karrier Load-Star 5 ply 'C' rated 205/75/15 ...flawless .
I think if you go up in size to a 15", it might have an issue fitting the wheel well with the Dexter Torflex torsion axle because the wheel/tire pivots as you add weight , it flexs into the well as you add weight to it . It might not matter as much if you have a leaf spring axle. check into that if you upgrade the tire size.
pics, will scroll of the diff. axles I think your size is 14"
http://www.etrailer.com/Tires-and-Wheels/Kenda/AM32153.html