Hi
What i would do if i wanted to run 2 aux batteries one in car other in trailer is use a red arc vsr from main crank battery to the car
aux battery then run a 6 mm twin down to thhe rear trailer battery with a bcdc redarc charger to up the voltage to charge those rear batteries well. and put a cole herse isolator in the van set up so that is connects the 6mm straight to the trailer batteries and isolates the bdcd. for 2 reasons if the bcdc packs up you can bypass it and so that you can have the car hooked to the trailer you can use the car's aux battery for more capacity. This also allows you to just plug it all in the charger turns its self on the main aux battery is getting charged and
no matter how flat the trailer batterys are the charger will only draw it's 20 amps with any lateish model patrol alternator can easly maintain.