Hi. In any car or RV, there are always "robber circuits" that take battery current when the batteries are connected. Most RVs have 2 battery systems...the engine battery and the "house" batteries. It sounds like your engine battery is the problem. Disconnecting the house batteries will not disconnect your engine battery. I installed one of those battery-disconnect
switches on my engine battery and turn it off when the RV is stored. It works well. The kind I like is the type with the round screw-down nut on the top. As far as what is draining your battery, it could be a light bulb, a dashboard clock, an engine computer, etc.