I just set up our 2014 Ford Taurus to flat tow. Had a 2013 Ford Fusion Hybrid I flat towed for years before that (until it was totaled in a wreck about 2 months ago). Both vehicles have worked great.
In both cases, I wired a charge line. Did it pretty much same way in both cases - used the 12V line and ground from 6-round wire/connector going to the car from the Motorhome (with a 20 amp fuse on the positive wire right where it connects to the car battery).
Had no choice with the Fusion Hybrid - battery would be dead in 2 hours of towing if didn't. Wasn't sure if Taurus would be same way or not, but since have to leave ignition in ACC mode when towing, odds are there is some drain on the battery, so I wired a charge line.
I avoided vehicles like the Fiesta and Focus, for this very reason - do not like at all, the idea of having to disconnect the battery. If only issue is battery running low, it is muuuuch easier to just run a charge line than have to connect/disconnect battery all the time.
I have indeed read about folks that did not disconnect the battery properly with these vehicles (believe it was a Focus), and a badly burnt transmission was the result. If you have one of those vehicles, I think you really got no choice but to disconnect the battery somehow; the price/risk for failure there is too great. A master battery disconnect switch of some kind might work better, though.
