You can usually hear it humming on a quite evening.
If the house batteries charge but the chassis batteries do not, the problem is likely a charge relay not closing. This may be intentional or a failure of some kind. The chassis battery is normally isolated form the house system until the house batteries are fully charged. Then a relay cuts in to allow charging the chassis too. If the house batteries are weak, the charge controller may never close the chassis relay. Or the relay itself may be stuck or just plain worn out.
Lou (aka Porky) is an expert on the Bounder battery charge control, so maybe he can provide more details.