Well,I would probably choose two group 27 or 31 AGM’s, and build a box that can double as a step up into the cab-over bed if possible. Those type batteries will fit in a step/box with a rise (height) that will feel normal to you when you are using the step. 6v golf cart batteries could be used if you want, But they are taller than the group 27 and 31 batteries, so if you made a step box for them, it would probably feel unusually tall to you.
Flooded batteries always need a sealed and vented compartment, but AGM’s aren’t as prone to that as wet cells. Technically, I believe all batteries can vent under the right conditions, but it’s pretty rare for AGM’s. They are more expensive than flooded deep cycle batteries, but if you compare quality flooded deep cycle to quality AGM deep cycle, they aren’t that much more.
:):)