2oldman wrote:
There's no need to ground it, unless something doesn't work without it.
It's true that you don't HAVE to ground it, but it also doesn't hurt anything if you DO.
If it isn't grounded to the vehicle chassis, it's referred to as having a 'floating ground'.
You can't have the engine alternator send current back to the house batteries if they aren't grounded. You also can't use the house batteries to start your engine if the engine battery is dead.
Finally, if you have a floating ground, every light, fan, 12 volt outlet, water pump, etc. has to have two wires, one hot wire and one return wire going back to the batteries. If the house batteries are grounded, many of these things only need a hot wire and can be grounded to the chassis.