The TORKLIFT BATTERY HANGER under your TC is an excellent idea. You know first hand how precious compartment space is in the truck camper.
Using the hanger under the truck bed does not require any mods to an enclosed compartment for venting fumes or draining fluids. It is also out of sight for security reasons. Of course being a slide-on camper shell you may have to come up with a battery quick disconnects much like you shore power cable arrangement. You can purchase 4GAUGE HD 2-conductor cable with connectors and plugs from ETRAILER. Then after getting the TC in place just plug-in the external battery bank that is under the truck bed.
I am fortunate having my whole tongue area across the front of my off-road camper deck. I can sit four batteries across the tongue with ease.
Roy Ken