As I understand it....it's not only about "pulling the load", it also supposedly affect them if sitting without load. The stronger battery will always be charging the weaker one.
As lead acid batteries sit, even not connected to anything, they'll slowly loose charge (that I know to be true)...so the old battery might be loosing charge more then the newer one....and so that's why the new battery is always charging the older one.
So as I see it, it's all about maximizing battery capacity and battery life span...
IMO, if you're not trying to save every amp for maximum off grid time, then who cares if it's a little less than optimum.
If it were me, I wouldn't hook up an old battery...especially a cheapo marine/hybrid type to a new and very expensive Trojan golf cart battery.... but if they're both marine hybrids, then I would just do it since I'm not a maximum off grid RV'er.
I like duckface's idea of an isolator to help with the situation... the link is to a switch, that if set to "both" just connects the two batteries together so it's not really isolating...it just allows you to use one battery, then switch to the other when needed...but then you'd have to be sure to switch to both for charging....there are automatic devices I think for this, but prob not worth it for your purposes.
but
that switch or something like it I do definitely suggest so that when in storage you can disconnect the batteries completely to eliminate parasitic loads in storage and to slow the self discharge to harmful levels. First thing I do now is install disconnects on my batteries...I've had to replace way to many over the years form letting the sit in storage too long...
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