...and not to belabor the point; but if you're just needing one part just replace the one. Then ride on and be happy. If you're needing two or three though there's not that much price difference between buying what's in their originally versus buying parts for the three phase upgrade. It's a worthwhile upgrade if you're buying most or all of the same parts anyway. Especially if you have to shop eBay for awhile and catch bargains.
When I still had my 05 SEEG if I was riding in cold weather and had on the heated clothing, the heated seat, the heated grips, the spot lights, the radio, etc the stock charging system couldn't keep up. I'd have to turn off spot lights and radio if I remember correctly to allow the system to keep up. Trailer lights may have been added to that mix but they were LED so not much of an add.
There was, if I remember correctly, a 40 amp upgrade you could do on those easily. That helped but it still did not quite run everything.
My old 2000 Road Glide and the 02 Road King both have the 50amp three phase parts installed. You don't get the drop that happens in single phase so the power is continuous and it's better at idle and up without dropping off when the RPMs fall back down. Can run everything on the Road Glide, all the heated stuff, all the lights, trailer, radio with its external amp, etc and the charging system has no problem keeping up.