Vehicle battery discharge

I have just traded in my caravan and bought a 2007 Bessacarr E560 motorhome, which has a Fiat Ducato chassis/engine. I had a solar panel and second leisure battery fitted by a firm that have wide experience with Bessacarr motorhomes, selling them, servicing them and hiring out a small fleet of them. They told me that a problem with motorhomes is that the vehicle battery is constantly being drained, even when the control panel in the habitation area is switched off. They recommended that the vehicle battery should be disconnected whenever the vehicle is going to stand idle for more than a few days to protect it from going flat. (Apparently, the solar panel charges up the two leisure batteries, but not the vehicle battery.) The caravan and motorhome dealer from whom we bought our motorhome claims that disconnecting the vehicle battery is unnecessary. Can somebody please advise me which of these companies is correct?