Those RV/Marine batteries are really combo start/house batteries and will not work as well or last as long as the 6 volt golf cart batteries. The GC batteries are true deep cycle batteries and can be discharged down to the 50% level and then recharged many times with no damage (but less is always better !). If you try to discharge the combo batteries to 50% and then recharge too often, they will not last too long.
Just don't discharge the GC batteries below 50%. And keep an eye on the water level. Distilled water only please !
Tim