Swapping a split relay for a DC to DC charger

I am currently exchanging a split relay charger in my motorhome for a DC to DC charger as I am installing a new gel as the house battery and need to regulate the voltage more precisely.

I have a fused 12v system running various appliances and lights hooked up to the current (ha ha!) house battery, along with a 1000w inverter/ solar/ mains charger which also hooks up to the house battery and provides the juice… I would like to keep the current (?) set up and wiring as much as possible… The neutral on the house battery is connected to the chassis along with all the neural return wires via the 12v fuse board…

Current relay switch has a cable from the starter battery positive coming in, a cable going to the house battery positive and a small wire to the chassis.

Just trying to clear my thoughts…

Essentially can I just swap the relay switch for the DC DC charger and use the same wiring? (e.g. no neural wires other than the chassis)

If not I am wondereding if I can use the chassis as the neutral wire from the the starter battery to the DC DC charger? …and also use the chassis as the neutral from the DC DC charger to the house battery? …and if I can do i need to isolate the existing 12v circuit from the chassis…

Any thoughts and advice would be great

Cheers

Dan