What type of driving are you mainly doing? Got digital MPG instrumentation yet? Going tank to tank isn't ideal feedback for adjusting the nut behind the wheel.
Also: with access to lots of batteries, you could investigate an alternator-optional approach. There's a lot to be saved by not using gasoline to generate electricity. (Lots of threads around on that topic.)
Regardless: hope you
create a garage entry and start a fuel log so we can watch your progress.
Welcome to the forum.