Because you already have load range D in a large size the best option is to go with an LT tire .
The heaviest duty tire available in a 15 inch tire is the Goodyear Cargo G26 sized 225 /70 R 15 load range D .
Do a search on this forum by typing G26 in the search box .
There is plenty of info that is repeated .
But all of the info is there .
That specific tire is about 115 % more tire than the best ST tire .
Do your search " G26 " and all of the info is there .
The best money spent is to upgrade at the time you are making a changeover .
It is the cheapest method to keep you from roadside assistance .