Just measure the voltage at the secondary bank when charging
If the built in echo charger on the unit "seems" to not be working, either of two things are happening: the EC is dead or the battery is fully charged and its acceptance won't take any more current
(although the voltage should be at charging voltage).
Since the Freedom 20 is a great unit, if the EC really doesn't work, then if it was my boat, I'd simply buy a separate EC, disconnect the one on the unit and be done with it.
BTW, I have a Freedom 15 (1998) before they built in the ECs, so I use a Yandina combiner, same secondary bank charging concept
, but not current
limiting, however, the secondary bank acceptance determines the charge current anyway.
PS - Depending on how your boat is wired, if the secondary V berth battery is on a 1-2-B switch with your house bank, to charge it just use B, and then switch it to 1 or 2 when charging is done. I've always considered B to mean two things: Both and Backup!