If the toad battery and RV batteries are both charged, there would be little current flowing from one to the other. I use the 12 v wire in a Blue Ox 6 pin coiled cord between RV and toad and not had any problems in thousands of miles of towing. I also have auto reset circuit breaker in the line from RV chassis batteries to plug and cord, then another in toad from battery to plug. The CB is just to insure no problems when connecting/ disconnecting.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI