Have seen similar questions but not found a definitive answer, so here goes. Split charge and leisure battery fitted in my Volkswagen T5, and I want to run a USB charger, cigarette lighter charger and LED light set all from the fuse box connected under the drivers seat (with the battery). My question is whether it would be safe to ground all the accessories to the earth bolt under the drivers seat, where the leisure battery is grounded.
In most automobiles the metal chassis is the common ground (earth). All accessories should be connected to it for return current, even if there are multiple power sources. A typical car already has an alternator and battery, two power sources both connected to ground.
It may be easier to add new ground connections to the frame rather than wire them back to an existing ground. It is possible that the ground wire under the seat is not designed to handle the extra return current you would be putting through it, which would cause problems for all devices running on it. That said, drilling into the frame is not always safe either. (Where are your fuel lines and brake lines?)
Safety is about ensuring that the positive terminal of your leisure battery is never connected to the main battery. The leisure battery should have a main fuse and per accessories fuses on the positive side.
If your vehicle goes for service, remove the leisure battery. Put a permanent label on the main battery that the car has a second battery, as a favor to anyone who has to work on it an emergency.
I would not attempt this modification on a hybrid vehicle, electric vehicle, or newer vehicles. Their electrical systems may differ.
(A few old collector vehicles have the positive terminal connected to the chassis, in which case this advice does not apply. Although if you happen to have such a vehicle, you're probably aware of this fact.)