It should be under the refrigerator near the furnace - really close to the fuse/distribution panel.
Thor uses a WFCO™ #8945 converter/charger. The reviews and experiences I've found online are mixed, leaning toward poor. While it's supposed to be a multi-stage charger there are a lot of reports of it not moving propery between the stages resulting in dead or cooked batteries. Based on the reviews I've decided not to leave our Axis plugged in while in storage.
It seems that the Progressive Dynamics Model #PD4645V is viewed as a great replacement. It is not an inverter/charger though. Just a straight converter/charger.
None of the batteries in our Axis (house or chassis) are maintenance-free. Install these to make it easy to add water.
Flow-Rite MP2000 Qwik-Fill 2 Battery Kit
Be sure to get the version for 12-volt batteries, not the set for paired 6 volt units. There are single battery sets available for the chassis battery too.