If you are having problems with something draining your chassis batteries, I discovered recently that your alternator can be the problem.

I isolated the problem by using a clamp amp meter on the battery cable and found that removing the cable on the alternator stopped the discharge. After taking my alternator to a repairman he showed me how the field winding was showing signs of decay and leakage. Once replaced my problem went away.

Turns out that this alternator problem can cause many other problems as well. Weak and erratic charging being one of them.