If you have alternator charging the chassis battery, and you get anything charging the house batteries how you like,
this will let either charging system charge the alternate battery quite nicely.
Unfortunately if your genset only has 13.5 hours on it, it probably has not been properly maintained. Time to change oil and filters and load test it for a number of hours.
Look it up but if I remember right the right procedure on a battery that is very low is fill with distilled water to just cover the plates, fully charge, then top off (don't let the fluid touch the bottom of the fill rings). Then overcharge to reverse what sulphation you can and stir around the acid and new water. A drive is good for stirring up the electrolyte as well
It is worth trying out that existing battery and seeing what it has left after a recovery attempt.
Jim
