Running twin 110 spare batteries.

Not boat related as it's for my camper but same principle.

Van has a starter battery and 85amp leisure battery which are isolated
from each other so I cant flatten starter battery. However I need more
backup power so I am planning on going for a 110amp in each footwell
with a suitable vent under the floor. Cant really link this up to the
charging circuit as the amperages are all different (cant fit a 110
under bonnet) Is this correct?

So what I was thinking of doing was having the 110s wired in parallel
but charged from a mains charger every now and again when we are on
mains. This would obviously need to be plumbed in to the existing
camper 12volt system but I guess somehow I would need to isolate the
charging circuits?

Any ideas how I should set this lot up?

Thank you.