Losing coolant but do not know where.

[Madmoly340]

Hey guys I have a 2007 Pete 387 with a apu the engine is a Cummings which I have had mandate removed
Best thing ever. ANY WAY I am using a half gallon a day of coolant but can't find where it's going the engine does not get hot unless I'm pulling up a mountain and then the fan kicks on. And the low coolant check engine light comes on only when I am sitting still and idling I have look every where for the leak. Its not leaking into the oil from what I can tell and can not find any on the ground
Please help out someone who is totally lost
Thanks
Mike