That's the same thing I went through. Our Allegro Bus has the batteries way in back and the EC-30 controls the 10KW generator, which is way at the front. The 2/0 cable wasn't adequate to crank the genset without a big voltage drop. It would crank manually with no problem (although a bit on the slow side) but the voltmeter would drop substantially. When the autostart was on, the EC30 would begin to crank it but the EC30 saw the low voltage and assumed that the batteries were too low so it aborts the autostart and kicks it into manual.
To correct this I added a designated battery up front just to crank the generator. I then connected a Trik-L-Start to allow the house charging system to maintain it's charge. Now the genset does not have a voltage drop, the EC30 is happy, and everything works as designed.
I just used your basic automotive cranking battery for this. The most time consuming was welding up a battery mounting plate to attach to the front frame rails.