Battery Voltages vs Capacity
Zeph, you clearly have a problem. Putting an electronic ''load'' on a large housebank is almost nothing. You need to follow Calder''s instructions, using the engine (start disabled) to put a REAL load on the bank, while checking both circuit integrity and also capacity of the bank. No short cuts, please - read to understand those tests and give them a try.
BUT let me add one suggestion. A day or two before doing so, run a Equalizing Charge on the bank, trying to shake whatever sulfation has occurred. Then run your tests.