Problems with 2 battery set using diode isolator

I have a Chevy 3500 conversion van with two batteries separated with a diode isolator. Battery #1 is the standard cranking battery with a capacity of 75AmpHours ±. Battery #2 consists of 2 235 ampHour 6 volt golf cart batteries in series. The alternator is a standard Delco-Remy DR44G 145 amp. I have gone through 2 starting batteries in the past 2 years. Lucky for me, Interstate replaced them under warranty but the rep said that he suspected that the battery was never getting fully charged. With the new battery in place I measured 12.41 volts resting and 14.71 volts after starting the engine, voltage measured at the alternator terminal was 15.66 volts. After driving about 20 miles, I measured with the engine hot and got 14.80V at the alternator terminal and 13.93V at the battery (through the diode isolator).. It appears that I am getting almost a volt drop through the isolator (950 mV). My question is: 1. How is the alternator charging profile determined and, is there any of raising the voltage output, and, 2. Do these voltages seem reasonable as is? 3. Would I see any benefit in fitting a relay separator in place of the diode isolator? Thank for any input, JohnnyFryJohnnyFry

Diode isolators do drop the voltage by .7 volts. A relay type isolator does not.