To answer the OP:
Yes, the trucks are designed to fit 4 divers in the cab, and 4 diver setups in the cargo bed with extra tanks.
No real reason to stray from that.
When we have camera setups, we put them on the laps of the people in the back seat or passenger seats.
Everything else goes in the cargo bed, including regulators.
We learned a long time ago how to bundle the BC, fins, mask, wetsuit, and regulatorss into a handy package using the shoulder straps of the BC.
4 of those fit easily in the space behind the tanks.
Most dive sites are less than 15 minutes from town.
I enjoy talking to my buddies on the way to dive sites and between dives too, especially since we decide on dive sites partially based on whether there are dive trucks present at the time. 3 trucks there? go to the next site.
Your results may vary, but an extra truck seems like a waste of money to me.