The block heater on my Scenic Cruiser turns off and on, on the side of the box of the bed. On my coach it has a switch box there with an amber light on it. When the light is on means the block heater is on. I use it in the winter months when we have cold weather. The engine starts a lot easier. The outlet for the block heater is where the inverter and the AC 50 amp chord reels in. Plug is on the top of the compartment. As to the battery problem, it will not be the block heater since that runs off AC power. Check the two twelve volt batteries with a load tester and see if they are good or need replacement. I have changed the last two with commercial heavy duty batteries that Napa sells and they are maintenance free batteries, and I am very happy with them. They have been lasting five to seven years on my coach. If the batteries test good, then you have something draining them when you are not using the coach. On my coach the chassis batteries do not charge with the inverter/charger, they only charge with the big Cat engine alternator. House batteries which are four six volt batteries hooked up in series and they do charge with the Inverter/charger.
2002 Gulf Stream Scenic Cruiser
2018 Buick Envision Essence
Neway Freightliner chassis
Aventa II Blue Ox