There's a reason the manufacturer didn't build in a tank - a whole slew of USCG regulations
and ABYC standards go into effect for built in tanks
or those attached in an enclosed space. Ventilation, hoses, corrosion
of tank bottoms, tank hold downs, tank materials, ignition protection, etc...
The biggest fear is fumes leaking somewhere that they can be started by an ignition source. Your battery charger
didn't have to be ignition protected, but now it can be a potential source. The ABYC standards are quite complex regarding ventilation, but a blower is required and there can be no point of entry of fumes outside of the tank locker - it's got to be sealed. A vent is also required.
Gasoline fumes begin at -40f and are heavier than air, like propane
. If you can keep the tank outside, you're much better off.