You have discovered why the DIODE ISOLATORS are not used anymore for charging DEEP CYCLE batteries. The high current diodes in the ISOLATOR have a .7VDC voltage drop across them and when it is really important to have 14.4 VDC for charging the deep cycle batteries and only getting 13.7VDC due to the voltage drop then it will be the difference of three hours charge time to more than twelve hours charge time to get the deep cycle batteries to their 90% charge state.
Today they use smart solenoids instead of diode based isolators to get the job done. The diode based isolators work great with your truck standard start battery but not so good feeding a deep cycle type battery unless you got a lot of time on hands to get it charged.
This is probably why you are finding both an isolator and solenoids in your setup.
just my thoughts
Roy Ken
