Battery Leakage?

We have a 3 month old T@B S M@xx with a 1 week new 80Ah AGM Deka deep cycle battery. I have a battery disconnect switch installed on the negative/ground leads. I charged the battery full with the Zamp and again with a trickle charger. As we're packing up for Idaho, I notice it's only (?) at 12.5V and am concerned I have a leak somewhere (I suppose that's the argument for putting the disconnect on the + side, but still, I'd like to find a leak/short if there is one). Does anyone have similar experience and/or suggestions where to look? I'm not sure our dealer (20miles away) has this expertise, and the local RV dealer may not want to work on the T@B (even at $100/hr labor...). Thanks once again!