Actually there's LESS electronic stuff on an OBS
than on the Super Duties and other later trucks. Fuel is, of course, all mechanical. And that's not what draws on the batteries. With any vehicle, once the engine starts, it's up to the alternator, not the batteries, to provide sufficient current to run everything.
The major purpose of the battery (or batteries) is to start the vehicle, and what requires two batteries for our trucks is the fact that it has such a bigeffin' starter to turn (to turn such a bigeffin engine at very high compression), at the very same time it's powering the glow plugs, which are, in effect, eight big toaster heating elements. The battery numbers you mention don't tell us much; are they models of a particular brand? TBH, I don't know if anyone makes a single battery that produces the kind of CCA that our trucks require. It'd have to be somewhere in the 1600 CCA range. Even if someone made one that big, I dunno if I'd want to try to lift one into the engine compartment.
Oh, waitaminit; are those John Deere tractor model numbers? Problem with JD is, they keep changing their numbering schemes; ours is only four years old, and it's being built now under a completely different number. I reckon a lot of them either have much smaller glow plugs, or are at most six-cylinders, or, like mine, just have an air pre-heater and no glow plugs at all. So we're talkin' a lot less initial power draw.
"Where the quarterback dates the homecoming queen,
The trucks are Ford, and the tractors green
And a man's word means everything......"