Normally 2 batteries are parallel to 2 batteries in serial (I hope you can understand it). The 12V worn down between it. So 2 batteries are getting more discharged then the other two's. I know the truck is equipped with the Niehoff Battery Control device. But in my opinion (and the opinion of my mechanic) this is not good for the livetime of the batteries. Basically the state of charge of the batteries is different.
And I can let the lights and radio run till the 12V batterie dies. The truck just need the 24V to get started.
(Hope you can understand my explanation. Speaking english is easier as writing
)
In short, what you did is worse in almost every measurable way, electrically. It will probably be good enough for your needs, since the system is admittedly overkill to begin with, but hopefully people don't read your post and think "That guy said the 12V drop was bad, so I need to mimic his 'solution' to get away from it." You came up with something cheaper and easier to source, but worse.
It's better to draw (and charge) across more batteries, not less. I'm not sure why your mechanic would suggest otherwise. They set the batteries up correctly in the original wiring layout, and splitting the two circuits results in more imbalance, and more potential problems.
Yes, with your layout you can run the 12V stuff until it dies, but with the original layout you could run it 2x as long without dying. Also, every time you run a lead-acid battery "dead" (below a couple volts on each cell), you cut roughly 15% off its total capacity/life. Deep cycle batteries use special materials to try to combat that, but it's still really bad for the battery.
AGM batteries, like the original military batteries, are significantly better technology. They have lower self-discharge rates, lower internal resistance, higher current capacity, extreme shock resistance, and significantly longer lifespans. Despite how expensive they seem, even buying the actual military batteries will nearly pay for itself from the extended life alone. If you're going to swap away from them, a set of 4 smaller COtS AGM batteries (e.g. Optima "red tops") would have been a better solution (though still a significant reduction in capability).