The 15" wheel I recommend for your boat trailer is the Americana Galvanized Steel Spoke Trailer Wheel # AM20534. This has a 6 on 5-1/2" bolt pattern like what you're needing and we actually have a review from a customer talking about how this was the only one they found to work with disc brakes.
Attached is a review video as well as a photo of the review for you to check out. view full answer...
Nice ride! I recommend using a power winch like the Superwinch LT2000 Electric Utility Winch, Item # 1120210. This winch has a 50 foot wire rope, has a rated line pull of 2,000-lbs, and a plug-in remote. This winch is a high efficiency low amp draw motor that will need 12v power. Make sure to firmly mount it in a stable location on your trailer. The Superwinch is capable of pulling about 4,300-lbs of rolling load up a 40 degree ramp which will be more than enough for your Harley Triglide.... view full answer...