86 Glendale Sterling, Gel batteries

Converter is not an inverter, the solar charger is suitable for gel as I checked. To be clear there are two batteries under the hood (the one for engine starting and the one for house/Deep cycle). Its a huge pain in the rear that they designed this RV with only one house battery and its under the hood with no additional room, at any rate this house battery was the one I was referring to. I think hooking up the solar panel to a couple gel batteries than hooking up these parallel will be bad since the vehicles alternator when driving will be charging the house battery and will also attempt to charge the gel batteries if I plug these into the house battery, connected to solar. I would buy a lead acid battery however I would not install this under the bed due to the ventilation requirements. The best idea I have come up with is to hookup the gel batteries under the bed with a separate charger and solar system, than plug these directly into the motorhome's 12V distribution block. Where can I find the distribution block? Would it be under the bed with the small circuit breaker panel and charge converter? Or does this need to be plugged directly into the charge converter somehow? If so how can I plug these batteries into the charge converter without having them charge when im plugged into 120V? As mentioned I wish to charge the gels with solar

On a side note im also looking at changing the converter to a used wfco wf-8835 smart charger.