Hi folks

Can one of you electrical experts help me out on this:
My 85 Westy has a 30A relay in the empty aux. battery compartment that is
used to switch the fridge and water faucet pump.  My question is this: Can I
put in an auxiliary battery and use the existing  relay to charge the
battery, and rewire so that the appliances and lights come off the auxiliary
battery?
Why buy relay kits when we are already wired for this?
What am I missing here?

Alfred
85 Westy