I have a 25 ft TT with 15 inch tires that I bought used. I tried to replace the tires with bigger and wider because they were seven years old and I had put over 20,000 miles on them.
As it turned out, my wheel wells were too small. Therefore, the larger, wider tire would not fit.
So, I changed from C rated tires to D rated tires of the same size. They do ride harder because they run at 65 psi rather than 50 psi but they have performed well over the last 10,000 miles. Also, the D rated tires are rated about 20% over the weight rating than the axle.
Good luck