Here's
another one for half the price. This could just connect in parallel with the batteries, when put under load (>60A) by the controller the voltage would sag until the batteries started putting in amps to the controller too (up hills etc). When the running load was <60 amps some of the excess power would go into recharging the batteries again. Going to mains voltage AC then back to DC in the charger would be fairly inefficient and you wouldn't get anywhere near the 60A that these DC generators can put out unless you had a super beefy charger.