darealmccoy,
Adding a "charge wire" to the system of most toads is a great idea. It is needed in many because of a situation like yours, there is a drain on the toads battery. As far as what pin in the plug, most plugs have a diagram right on them, telling what does what. I used the largest wire I could, within reason from the pig tail all the way to the positive post on our '11 Honda CRV.
The heck with pulling those dam fuses. The one for the CRV is so hard to get to and, you have to be standing on your head and have triple joints in order to pull it and replace it. So, the addition of the charge wire was the order of the day. It's worked flawless now for several thousand miles.
Now, just how long would it take for and auxiliary braking system to run a toads battery down enough so the toad will not start, well that's quite variable. But, according to folks on here an other RV forums, sometimes it's rather rapidly. So, the charge wire is the answer. If you can do say, a 12 or even a 10GA wire to the positive post on the toads battery, from your pig tail between the two vehicles, you'll be way more than protected and won't have to pull any fuses. Your choice.
Scott
Scott and Karla SDFD RETIRED2004 Itasca Horizon, 36GD Slate Blue 330 CAT
2011 GMC Sierra 1500 Ext Cab 4x4 Toad
2008 Caliente Red LVL II GL 1800 Goldwing KI60ND