Unfortunately, I misunderstood your question.
How high is the voltage on the service battery?
If it’s just a matter of trickle charging over the winter, then the solution could work.
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Another solution that comes to mind would be to connect one of the MPPT devices to the Agrofets input for the winter months, perhaps with a smart battery sense.
However, these are all solutions that require a certain amount of expertise in planning and installation. On the other hand, they are also very inexpensive.