Phenrichs, the problem is the outgassing of Hydrogen gas during charging. Thus the need for good ventilation. The battery being mounted in proximity to the LP tank, in an outdoors environment, does not present an enormous hazard.
It is entirely possible to mount extra batteries "under the bed" in a box with ventilation to the outside. Here is box that I use and the ventilation method I chose. The white hose is the box vent.
Photo #1 is the box with two 6V CG2 Batteries.
Photo #2 is the outside vent from the battery box
