I am mighty confused!
I think you have 3 Ca batteries that require 15.5 VDC absorption voltage. Correct?
You have a Xantrex three bank battery charger that will or will not achieve the desired 15.5 VDC?
You have a separate Xantrex inverter (not an inverter/charger) that will not achieve the desired 15.5 VDC. Correct?
I think that you want to draw power from the Ca battery bank to provide DC to the inverter which will make AC to run the battery charger to charge the Ca battery bank. If this is correct, you will lose the battle as every time you go through an energy conversion process (DC > AC > DC) the system inefficiencies multiply together and doom you. Battery charger about 90% efficient. A good inverter, maybe 95% efficient. System efficiency = .9 x.95 = .855 So for every amp-hour removed from the bank, 0.86 amp-hr will be put into the bank by the system.
Unless I am missing something in your description which is certainly possible.
Regarding operating your nominal 12 VDC equipment at 15.5 VDC...not recommended. That is why, when we equalize a FLA bank, nominally to 15.5 to 16 VDC, one of the preliminary steps is to isolate the battery bank being isolated from the normal 12 VDC loads.
Hope this helps.