A ground is a ground, however the 12 V supply typically has much higher currents so the wire size might not be appropriate. But the best reason to not do this is that it contravenes code. If there is ever a fire, even if you are sure it has nothing to do with wiring, your insurance may not cover you. For a couple bucks worth of wire and drilling 1 hole, why risk it? Also do yourself a favor and run the 12V return to either the ground window or the actual battery, not the frame or you'll be messing with it a couple times per season for as long as you own the camper.