Better yet just unplug at the trailer connector.
Unfortunately, once our toad is all hooked up with tow shields, etc, in place, it's quite a chore to get at the cable to plug or unplug. Unplugging at the car end requires removal of the tow shield, and at the MH end removal of the bikes on the bike rack. Also some acrobatic skills are required that are beyond my old bones. We try to have the toad all hooked up the night before we leave, just to make departure easier. So, it's connected overnight before we roll.
As far as the relay, I'll have that explored next time we get near our regular RV service facility. Hooking up a diode I can probably handle myself. Not sure about a relay.
Thanks
al