[Brad Townsend]

On my coach: When plugged in to shore power I leave the battery merge switch on. The shore power only charges the house batteries I have confirmed this by checking the voltage with a meter, when I plug in only the house batteries show a charge taking place. I find this also keeps both sets of batteries at the same level which is useful for boondocking when I can also merge the batteries for double the usable capacity but if this is done one must also make sure they have the ability to fully recharge both house and chassis batteries. This could be taken into consideration when outfitting solar panels for dry camping (bondocking).
I think that using the merge switch in this manner could make it last longer not sure but years ago I was told by an industrial electrician that things with electrical contacts are best left on than switching on and off. This was in reference to a large boring mill where I would have problems with the starting relays. After leaving the main power on I never had a problem again.
I was reminded about light bulbs, when do they blow out usually when they are turned on. The stress of on and off on the element weakens it to the point of failure.

You could check the voltage of your batteries before plugging in to confirm if this is the same on your coach.
Thats what I do with my rig.
Brad