I'd skip the parallel battery hookup in favor of a switchable arrangement that only draws on one battery at a time. Batteries degrade at different rates, due to age, manufacturer, chemistry, group, etc. If you gang two or more batteries in parallel, they will attempt to maintain a constant voltage across the whole set. That means the strongest one will lose voltage "recharging" the weaker one(s). By hooking up to one battery at a time, you get its full load. When the first one is depleted, switch (manually) to the next battery & use everything it has.