Look high and low for a 'battery shut-off' switch. It's common on older units with a single-stage 'battery boiler'-type charger, to permit leaving them plugged in without damaging the house battery.
Some use a relay and small switch on the monitor panel, some use a large 'clunk' direct switch in the main house battery lead, often near the floor.
Jim, "Mo' coffee!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory')