A quick Google search gave me this info:
Yes, you can use a bare ground wire for an off-grid 12-volt DC system. In off-grid systems, the ground wire is typically used for safety purposes to provide a path for electrical faults to safely discharge into the ground. It helps protect against electric shock and can also help prevent damage to equipment in the event of a fault. The use of a bare ground wire is common in DC systems, where the ground wire is typically connected to the negative terminal of the battery or power source.