I just use a 4' cable I made to connect the aux battery to the main
battery for an on-board jump (aux is under rear seat).  I didn't reply
to the question because I do not have the exact set-up you have and
wasn't sure how yours would do.

Would this still be okay if one had a deep-cycle battery?  I use a
deep cycle that is coded 'DP' for dual purpose, i.e. starting &
trolling but would one that is a true 'DC' deep cycle be bothered by
the sudden heavy draw used to crank an engine?