Flat Batteries

Hi - our recently purchased Chausson suffered two flat batteries recently, engine and service. I found the cause - I hope! it was the reversing cameras (two off) junction box under the dash. Found it uses approx. 0.5 amps when working and 0.3 amps when cameras off. It is powered from a permanent live+. There is no auxiliary position on the Ignition switch. I also have a Sterling battery to battery charger as well as the factory 240V AC charger. My question is - does anyone know if the engine battery is drained down, will the service also drain through the Sterling battery to battery charger? Everything was turned off including the service battery isolator. If anyone can enlighten me to the mysteries of this system I would be very grateful