should there not be a feed should there not be a feed from the negative side of your leisure battry bank back to the chasis. is your voltage sensing relay ajustable? if the trip voltage is set too low it will leave the leasure batteries connected to the starter battery once the alternator stops spinning. the different brands/capacities/discharge rates could certainly cause noticable drain when there is no alternater feed comming in. had some thing similar on a friends van with a voltage sensing relay, exept it was out in the other direction and wasn’t letting charge through when the engine was running so the leasure battery was loosing charge and not charging back up.