c-max and RVI
I've never had 12V battery issues with my c-max so the issue may not be across the board.
But, I agree, either run a charge line or use an external battery pack. The latter option has worked fine for me. A 155Wh lithium pack runs the rvibrake for a full day of towing without issues.
The only thing I did was replace the cigarette lighter adapter with a SAE connection to the battery pack - much more reliable.
I'd also reiterate that you need to pull the vacuum pump fuse before towing. Some people say they tow fine without doing that but many, myself included, found the pump can come on even when the car is off and cause the c-max brakes to lock up when the rvibrake engages.
Nothing more fun than hearing the scream of brakes behind you while driving in heavy traffic... ;>)
Phil & Ruth
2020 Fleetwood Discovery LXE 40M
Discovery Owners Assoc. Inc. (DOAI)