It would take several hours and many pages to completely describe how I have added self-jump-start capability to our trucks; I will try to hit the high spots.
First, each truck has THREE Group-31S batteries, TWO for starting and ONE that handles ALL accessories.
This set-up is not nearly so complicated to install as it is to explain.
TWO batteries connect to the big starter cable and ONLY that cable, plus the push-button start and solenoid-relay power of course.
ONE battery has ALL other accessories connected to it, EVERYTHING.
The alternator charge-cable connects directly to the START battery circuit.
A big constant-duty solenoid/relay connects the ACCESSORY battery circuit to the alternator and the START batteries ONLY when the engine oil-pressure exceeds 5-PSI.
Plans are to install a second solenoid that will isolate the START batteries from the alternator as well to protect the truck from catching fire should an alternator short-circuit when no one is around; this happens more frequently than one might think and is the cause for many a house or barn to burn to the ground.
A BIG battery-switch between the START circuit and the ACCESSORY circuit provides the ability to self-jump-start the START circuit, in the event that something should happen to the START batteries.
A second BIG battery-switch has the ability to completely dis-connect the START batteries from the circuit, should some internal failure be causing a battery short-circuit.
I can leave any electrical system ON until the ACCESSORY battery is completely drained and have no effect whatsoever to the START batteries.
I devised this system after about the tenth time my wife left something ON and stranded herself; since this installation, I haven't had to go fetch her a single time.
The START batteries seem to last much longer and remain much stronger when they are isolated from the ACCESSORIES.
As for jumper-cable starting, it has a lot to do with the jumper-cables.
Those wimpy discount-store cables may work for a Datsun B210, but they aren't gonna cut it on a big diesel engine.
My cables are 2-AWG and it takes both hands to squeeze the clamps open.
The clamps will almost bite off a soft lead post.
I have no problem jump-starting with these cables.
If one is going to go to all the trouble of carrying/storing spare batteries, why not permanently mount them and wire them up as I have thus described.
Besides all the obvious advantages, my method also keeps the extra batteries in a constant state of charge.