You can ground the second battery to the frame, so long as it's a good ground - using a star washer and silicone grease will insure a good ground and prevent oxidation (corrosion) that could otherwise cause problems later. The True will most often be grounded to a negative terminal of either battery - - but again, ground is common, so as long as the ground is and remains good, doesn't matter where it's attached.
When adding small accessories and electronics, good practice is to run the ground back to the ground buss - and the buss is connected to a battery negative. The problem with grounding a bunch of different things to the frame in different places is in maintaining all those grounds in pristine condition to prevent resistance build-up, "floating grounds" etc. A ground buss bar (often as part of the fuse box) keeps things in order.