I personally haven't looked at 15" LT tires, though others have found and posted before. So there are some...
But I personally wouldn't bother with them, unless it were the ONLY option.
16s have got so much more to offer in reliability, availability, and price points that I just don't see any reason not to use them if at all possible.
When I bought my latest TT, It had STs, 15" LRD on it. I towed it home from Sylvania Ohio to Phoenix AZ, being care not to exceed 65 MPH all the way (what slow trip back that was) and immediatly swapped the tires from my old TT over to the new one.. Rims and all.
I would much rather have 4 year old Michelin LTs that any new ST tire.
That mean't giving up the shiney aluminum rims as My old tires (Michelin XPS RIBS 16" LRE) were mounted on white spoke rims.
I actually like the look as it grows on you.
And I'd rather have reliability than looks anyday.
The aluminum rims and tires with about 2500 miles on them still looked new, and was a major selling point to the new owner.
I do think that the Marathons you chose would be a easier sell than than the other choice.. Goodyear is a very well known name.