That is very interesting. You would think that the fuse protected voltage supply side including the converter, battery systems (vehicle & trailer) and breakaway switch could be designed so that they are not affected by the isolation switch. Battery charging could take place no matter what the position of this switch. After all, the idea behind this switch is, I think, to keep from slowly draining the battery down via 12V DC detectors and other monitoring equipment, etc. when not in use and another power source is unavailable. I know it is standard practice to disconnect the tow vehicle electrical from the trailer when parked up and on 120V AC. So this isolates the vehicle battery from being possibly drained.
Based on your discussion I went ahead and fashioned a spacer which fits behind the knob and keeps it in the open position while towing or when need be. I simply took a thick stiff elastomer gasket and split it to fit nicely over the stem behind the pull knob. The only problem is it will probably grow legs over time!
Thanks for the discussion.