If it were my boat
I would install an Echo-Charger from Xantrex. It is simple and has always been reliable in the boats that I have installed them in. They are a snap to install and only use small (10-12 AWG) wires.
I wouldn't recommend a battery isolator as there is too much loss from the diodes. You could use a combiner but you really don't need it for what you are doing and you would need to use much larger gauge cables
I would recommend having a parallel switch so you could start the genset with the house batteries
if the genset start battery goes dead but you should not leave it on unless you need to do that, and then turn it off. A combiner could also parallel the batteries if there is a manual override if a charging
source is not available. The combiners only combine when there is charge to the banks. (Or - you could use jumpers in an emergency
if they are close to each other). Some gensets will charge their own start batteries.