Replacing an old converter ... what's the "Towing Vehicle" circuit for?

[elmer]

I accidentally shorted out a circuit on my old 1976-era TU-331W UniVolt, and even after replacing the blown fuse, the unit now sounds different and has a nasty 60Hz/30Hz ripple on the output.

So that's the final nudge that I need to replace it with something more modern.

I've bought a PowerMax PM4-45 and a new fuse panel from Amazon (not sure what the difference is between Amazon's tandard PM4 and the PM4B "boondocker" that seems to be recommended here, except for the extra year of warranty).

My question is about the wiring in my Argosy.

There are 3 20A circuits that make sense to me, and should be fairly-easily wired into the new fuse panel. But there's also a 4th 30A circuit marked "Towing Vehicle" in the old converter.

Is this there to charge the vehicle battery from the trailer, or is it there to have the vehicle battery power the interior lights in the trailer? Or is it something else entirely, like the emergency tear-away brakes?

I'd rather like to know what it's for before I wire it up into the new fuse panel!

Thanks!

[TG Twinkie]

The 30 amp fuse is in the "charge line" from the TV. Your TV keeps the "house" battery charged while driving.
There may be another 30 amp fuse mounted separately on the inner skin. It is in the negative circuit from the battery.
TV = tow vehicle

[elmer]

Ah ... OK, that makes sense.

I couldn't immediately figure out why it would be wired that way, but I get it now.

Thanks!