Towing - Is the trailer battery wire usually isolated?

[audiocontr]

This is not directly pinz related...

I own a 01 F250 diesel that i use to tow my Pinzgauer. The 7 pin connector has a 30 amp "trailer battery charging circuit" wire that i use to keep the small break away battery charged on my trailer. Does anyone know if this wire is normally isolated from the car batteries beyond a large relay? With all of the discussions we've had of undercharging/undercharging our batteries, it would seem that I'm in the same situation with my diesel... basically under charging my truck batteries when the trailer battery is charged.

A simple diode isolator would give me about 13 v back there when the car is running, yet isolate it from the main batteries. Have others done such a thing?