I just installed a US Gear Universal Tow Brake system that came with the used Honda Element that I bought for towing. The simple charge wire that you described is exactly what they recommend and that's the way mine's configured. Works fine!
One question in my mind is what to do when the toad is still connected to the parked RV. Do you leave the umbilical cable connected in that instance? If it's connected the toad battery and the RV's chassis battery are in parallel. I guess that shouldn't hurt anything, but I've been disconnecting the cable for an extended parking period - overnight for example.
Coyle & Sue
Cassity (Labrador) and Rylee (Standard Poodle)
2007 Damon Challenger 355; 2003 Element toad