Battery Isolator

Mexicowanderer is correct....you've got several issues going on here. Your voltage regulator might be faulty, or it simply may have a resistive ground. Diode style isolators are not the greatest and if you do want to go with a battery isolator, a MOSFET based isolator by Perfect Switch in San Diego can handle far more current than your alternator will generate and do so with no heat sinking and less resistance than a foot of #4 cable. Mexicowanderer is likely unfamiliar with newer MOSFET-based isolator technology, but I'll tell you....it's come a long way since the old sure-power days. One thing that most folks don't appreciate is the importance of ground connections. On my install, we always directly couple the alternator casing to the ground of battery A and the ground of battery B. Do not depend on chassis or multiple mechanical grounds to provide reliable, low resistance connections. Poor grounding creates charging imbalances which in turn causes over-charging or under-charging issues. I would start with the easiest thing first...improve ground paths, then check your charging voltage from alternator to isolator input, then isolator output to each battery post. With a good meter and assistance from someone who understands auto-electrics, you should be able to quickly determine if replacing the voltage regulator and/or isolator is necessary. Simply throwing money at the problem and playing musical parts is not the best way to sort out this issue.