Hey Connor,
Load range D 15" tires should help quite a bit. They don't have to spin as fast which should generate less heat and also help the bearings last longer.
Although my single axle EZ Loader roller trailer is officially rated at 3100 lbs load capacity, I believe the rating is pretty conservative because my fully loaded weight is in the 3400-3500 lb range and I've towed it all over the state with zero tire problems in the 27 years I've had it. Have made numerous trips to the keys towing on I-95/turnpike @ 60-65 mph. Tandem trailers with the same roller configuration are rated for 4000 lb boats, so the 25 rollers supporting the hull are definitely not the limiting factor. The trailer is HD hot dipped galvanized steel and weighs about 900 lbs, so my towed weight is in the 4400 lb range. It has a 5000 lb axle on it with 6-lug wheels. The tires are Denman ST225-75D15's, rated for 2540 lb max load at 65 psi, so they're are only loaded at about 87% of max capacity, which is probably the main reason I've had no problems. The tires are bias ply, 4 ply nylon in the sidewalls and 5 plies in the tread. If you had 4 tires like that, you'd have over 10,000 lbs capacity, so they'd only be loaded to around 70% of capacity, so I'd think that would work fine.
Since I never dunk the trailer, I ran 12" drum brakes for years but finally switched over to Kodiak disks because the drum brakes were such a pain to keep adjusted evenly. I didn't need the stainless version, but obviously you'd want that on a trailer you dunk in saltwater. This pic will show why I prefer a roller trailer!
http://i188.photobucket.com/albums/z...s/IMAG0090.jpg
I weighed all the gear on the boat one time and created a spreadsheet I'll e-mail to you - it's amazing how all that stuff adds up! Denny