What did you do to your Class C MH today???

Went to start it to run it for the monthly run and DEAD battery. I've had this problem since we got the unit. So I plugged it in and waited a few days and walla the engine battery is charged. I've had the unit for almost 2 years now and this is the first time i tried to charge the battery this way. I usually put my portable charger on the engine battery. I have also completely disconnected the cable that runs to the coach battery's to see if they were draining the main battery. but it still drained. I know it being a 012 there are sneak circuits in the ford chassis. My question is does the on board charger that is suppose to charge the aux battery's suppose to charge the engine battery? puzzled?