GMC Repair: aux battery not charging, gmc 4x4, ohm meter

Question
i have 00 gmc 4x4 ex-cab 6.0L. the aux battery doesn't charge. the main battery is charging fine. noticed the problem after i seen the aux battery came lose and hit the hood it had burn marks on hood, looks like it hit it allot. from the factory they do not give much room for the aux battery. i fixed the problem of it coming loose. and also gave it more room if it does come loose. one more thing it appears that the tow/haul is not working also. is there an in line fuse in the system? i checked all normal fuses under dash and under hood.

Answer
Hi
From the generator, the charging system for the Aux. Battery goes THROUGH the starter, through a fusable link (probabally right at the starter) through an "Auxriallary battery charging relay, through a 125A Mega fuse, and to the battery.
If the battery was shorting on the hood, I would HIGHLY suspect the 125A mega fuse (large black fuse)and I think it is located right next to the  aux. battery close to the A/C accumulator.
You can check fuse with an Ohm meter.
Hope it helps
Pawl