This is maybe my ideal battery setup. 

Under the rear seat, larger engine battery and large deep cycle battery
Under the front seats two deep cycle batteries that will fit - Frank of Canada had a part number for oem type

Then a gray HD switch from Bluesea to switch between the engine and the house batteries
Then a second likewise HD switch between the large deep cycle and the two-smaller-wired-in-parallel batties. 

This way I can choose to start my engine from the engine battery or the larger deep cycle. 
I can also choose to run an inverter between either the large deep cycle or the two-smaller-in-parallel batteries. 

That last bit gives me a chance to let the battery rest and properly charge while I continue to use the two-in-parallel, and then vice la versa.