Electrical problem..Magnatec converter?

I'm thinking that is a correct assumption that the converter/charger is drawing too much current for the storage building's GFI outlet. Given, that is, if one bad battery sucked the others down. I doubled checked most everything, a few minutes ago, including the numerous 'basement' lights to make sure no switch/light/appliance was on. The only things, I believe, that may be constant are the house monitor and car radio memory. I'm inclined to think the engine battery is most likely the culprit, since it's old '06'! I hadn't realized the 2 house and 1 engine battery were somehow interconnected. It's a semi-major project to get the engine battery out and will have to wait a few days for warmer weather. Left my battery charger on 2 amp deep cycle mode. The other 'thingy' I've never fully understood is how the big round 'Master' switch functions. It is located next to the driver's seat. There a couple of heater things...flat black rubber.. hanging over the black and gray water valve areas. Perhaps they could be thermostatically controlled from somewhere and were 'on' during this cold snap we're having?