Hi Sting,
Well, I've never claimed to be an electronics tech... so I tend to go by what "those in the know," tell me when it comes to that realm. Unfortunately, neither do I have a photographic memory. So the exact reasoning and explanation given to me at that time (late 1991) escapes me, but I do remember it making sense... didn't have a wonderful HTF to turn to with these questions!
The trailer battery was frequently discharged (no doubt because it wasn't being charged due to the blown fuse) and probably blowing replacement fuses because of the current surge when I plugged into the TV. For years I gave up on charging the trailer battery while underway. I'd get 5 to 10 top raisings out of the battery and would then put the trickle charger (we always carried) on it and charge it up. So you're probably correct about the heavier wire connecting the TV battery and trailer battery. I always thought the isolator had more to do with. Guess I was fed some bad info... my apologies if I've mislead any readers.
Bottom line... whatever fixed it, I was happy!
Jim