Just got off the phone with "Jesse" at Big O in Kennewick.
He's agreed, although somewhat reluctantly, to replace the one cracked LT tire, which I said was ok with me. He wanted to replace all 4 with STs, but not for free. The OEM sticker on the coach specified LTs, so he said ok.
I guess what this comes down to is the personal preference (and maybe store policy) of what kind of tires to put on a trailer.
I was getting nowhere with emails. Sometimes a good old phone call is the best way to deal with some businesses. He apologized for this taking so long, which I appreciated. It shouldn't be this difficult.