So I was going to put a DC-DC charger between the auxiliary battery in the back of the Jeep and the 105AH in the trailer, as after hooking it up I was loosing 1.3v at the trailer battery connection. Then I realised the Anderson plug to battery clamps that I grabbed from my solar panel bag were really only meant to handle 10A and were way to small so it wasn't a good test.
I made a proper cable to go from internal trailer plug to battery and now I get 13.8V at the trailer battery when there is 13.9V at the Aux battery in the Jeep. So no need for a DC-DC charger in between as this is really only to keep the battery fully charged while on the road to camp as solar will be plugged in when I set up and the battery will be fully charged when I leave home. That's a few hundred saved.
Next to put all the plugs and things on the battery box so it is self contained.
Voltage at the trailer battery
Voltage at the starting battery and second battery setup in the Jeep.
Cheers
Steve