You may have a parasitic drain killing the battery. Disconnect the NEGATIVE cable, set your VOM to Amps, and check between the negative post and the cable you just disconnected. 50 milliamps is the normal maximum drain by standard, if there is more you have to isolate that device by process of elimination.
I had the same problem with my Class A with both batteries. I found the rear camera was draining the house battery and a courtesy light was draining the chassis battery. The latter was a mystery, since no light was on anywhere. I pulled fuses until I got a zero amp reading in both cases.