FWIW I had a similar situation with a 1982 GS1100GK. I had JUST replaced the battery with a brand new one, left it on charge all night, started vacation the next day. I had to jump start the bike by the end of the first day. Continued down the Blue Ridge Parkway. On the way home, on I-81 right outside Martinsburg, WV, the bike totally stopped AND blew fuses. The verdict was a bad battery AND regulator rectifier!
The bike shop in Hagerstown, MD swapped a used regulator and charged the battery all night. The conclusion was that the original charge did not fully charge the battery. Hence, the regulator rectifier and charging system was working overtime to finish the job, eventually overheating and ruining the original RR in the ensuing days. Do I believe it? Not 100%, but I'll never know for sure. So that's my story, I'm sticking to it!