FYI - my understanding of grounding in general is fuzzy, so take this with a grain of salt. In a stationary home, be it a "mobile" home or stick-built house, grounding is essential for AC currents and there is a permanent rod into the earth for grounding. In a truly mobile situation, like a camper van, it's literally impossible to actually create a true ground. After all, your rubber tires insulate you from the earth very well and you can't exactly drive a rod into the ground and drive around. So the "ground" in a 12v mobile like a van isn't really a ground. There is some limited benefit to creating a "ground" through the vehicle chassis, but exactly how that benefit relates to safety I don't really understand yet, even though I've been trying to learn it for years (albeit not very hard). The main reason people connect to the chassis is to create a sort of massive negative "bus bar". Instead of running - negative/black wires everywhere, from battery to the loads (pumps, lights, etc), you only have to run one wire, the + positive/red wires. If the vehicle had a separate isolated chassis, then you could use each chassis for a + and - "bus bar".
That said, generally, if your device's user manual doesn't instruct you to apply a ground wire (some have no such place to put one), then don't.
Also
@TacomaJoe is spot on about how to attach a wire to the chassis. Bare metal and do not rely upon the screw for conductivity. Heck it could be a plastic screw if it was strong enough plastic like Delrin to withstand the forces of road vibrations! Also, do not place a washer or anything between the ring terminal and bare metal. Direct and solid contact, with enough physical compression from the screw to really push the two surfaces into each other. Normal tightening with regular hand tools is fine, you don't need to pull out the 1/2" air impact wrench. In a high vibration place like a van, it's a good idea to check the screws now and then to make sure they are still tight. Or use lock washers as those work pretty well, too. Stainless steel screws and washers are handy if in a marine or high-moisture environment. In such an environment, it might be a good idea to put sort of ox-guard or even a very thin coat of petroleum grease on the contact surfaces to reduce rust.
