I hate to get in to this....knowing the angst of it ......however, are there no 15" LT tires that can go on a TT?
Sure. We have a few sizes and a couple of load ranges in a 15" LT and all position cargo/van/trailer tires.
Depending on room in the wheel well and proper wheel pressure/wheel width requirements the Goodyear Wrangler HT in a LT235/75-15 C (50 psi) at 1980 lb capacity. Most LT tire makers carry this size.
And a Wrangler HT in a LT215/75-15 D (65 psi) at 2090 lbs capacity.
Maxxis LT U-168 are a commercial grade tire with several 15" sizes and have became popular with all types of trailer owners.
Vanco 2 from Continental another popular tire for trailer users has several 15" sizes and load capacities.
I see the Yokohama RY215 in a 700R 15 D at 2040 lbs capacity is popular on some of the other RV websites.
Do your home work on the numbers.