That's basically what am asking him to do asprin624 I have been an Electronic Tech for over 50yrs now, etc etc etc long story + IT have also been using Multi Isolated Battery systems "45yrs" before they invented the fancy crap they have now, and I still have to help people out with failures and I just hook them up a basic system " ie :bypass the crap with simple devices "and away they go very happy.
This is the way.

I will add to Biggerdave's post :
A car battery is rated as Crank Amps " the higher the better" if you divide that CCA by 7.25 you get the AH rate which ain't much. CCA vehicle batteries don't last long as Deep Cycle Batteries and if you let them sit drawing Current they go flat quick, if you try to use a Deep Cycle battery as a vehicle Starter many times it will also not last long. Difference between CCA and Deep Cycle is the Plate thickness CCA have thinner plates to pass more Current short term. Deep Cycle Batteries are thicker to allow longer current drain over time. I have a combo of both in my vehicle, I have a 720cca for main and Secondary is a 100A/H "725CCA" as my long draw Battery for running Fridges/lights/inverters + Solar recharging and if I use my Vehicle battery for internal lighting I bypass my Battery isolation relay to give the main a boost charger before starting the Vehicle as I have had Speedo trip meter/radio reset to basic. eh.
This also the way.