Dethleffs are fitted with a sophisticated charging and control system from Schaudt. It maintains starter and hab batteries to optimum charge by mains electric or alternator also solar by the edition of an LR128 or mppt controller which is plug and play (you can find details on the internet) Solar panel just plugs into these units so connections are at this charger unit often under the passenger seat.
Never connect additional charging systems i.e solar direct to the battery it undermines the existing system and can degrade battery life by over charging. Regarding hab battery accessible by removing the seat which is very heavy and has connections for seat belt emergency controls.