Lifepo4 backflow question

I installed a lifepo4 battery in my trailer. With the 7pin connected, and the car in idle parked, it will send about 8 amps to the lifepo4 battery.

but with the car in drive it reverses and the lifepo4 battery sends about 2 amps to the Tow vehicle battery

I assume this is due to the TV battery being used and wanting a charge.

How can I stop this backflow? Will a DC to DC converter do the trick? Will the converter ensure the lifepo4 battery always gets a charge? Or will it simply end up being zero amps and acting like a diode?

Thanks!