Your idea is very ingenious, but I'm guessing that you haven't seen the numerous threads on this website about the simple answer to this problem. Run a charge line through your harness from your motorhome to your toad. After towing several different cars over the years I was recently made aware of this solution and now I swear by it. The motorhome alternator charges your car battery while you tow, the same way it charges the batteries in your motorhome while you drive. This eliminates the need to pull any fuses, or install any type of fuse cutoff switch. Simple and inexpensive, what could be better?
Just sayin'
Happy travels.....
Bill