Yep and to eliminate the mess entirely use what Mr. Pianotuna suggests in the form of a
Sure Power Battery Separator
Part #: 1315-200
The Sure Power Battery Separator is designed for use in multi-battery applications as a excessive loading while allowing auxiliary batteries to be charged. The Sure Power Battery Separator has two basic operational characteristics.
Bi-Directional Battery Separator :
Able to have two charge sources connected; one upon the Main Battery, and a second upon the Auxiliary Battery - This means that the charge current can go in two different directions, from the Main Battery to the Auxiliary Battery, or the Auxilary Battery to the Main Battery. Whichever battery is connected to the ACTIVE charging source will be considered the "Main Battery" as far as the functions of the Battery Separator are concerned.
Voltage: 12 Volts
Amperage: 200 Amps
Approx. Item Weight: 3 Pounds
https://www.allbatterysalesandservice.com/graphics/INSTRUCTION_1315-200.pdfEliminating wires, switches and junk grade relay/solenoid will eliminate about 99% of the chance you will have a charge interconnect problem for the next (many) years. Solenoids with switches are buggy whips. Vulnerable buggy whips. Yuck.